Light emitting device, driving method for the same and electronic apparatus

ABSTRACT

It is a problem to provide a light-emitting device capable of obtaining a constant brightness without being affected by deterioration in an organic light-emitting layer or temperature change, and of making desired color display. The lowering in OLED brightness due to deterioration is reduced by causing the OLED to emit light while keeping constant the current flowing through the OLED instead of causing the OLED to emit light while keeping constant the OLED drive voltage. Namely, OLED brightness is controlled not by voltage but by current thereby preventing against the change in OLED brightness due to deterioration of OLED. Specifically, the drain current Id of a transistor for supplying a current to the OLED is controlled in a signal line drive circuit thereby keeping constant the drain current Id without relying upon the value of a load resistance.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.11/139,441 filed May 13, 2005, now U.S. Pat. No. 7,106,006 which is acontinuation of U.S. application Ser. No. 10/715,472 filed Nov. 19,2003, now U.S. Pat. No. 6,914,390, which is a divisional of U.S.application Ser. No. 10/101,628 filed Mar. 21, 2002, now U.S. Pat. No.6,661,180, which, under 35 USC 119, claims the benefit of a foreignpriority application filed in Japan as Ser. No. 2001-081779 on Mar. 22,2001.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an OLED panel having an organic OLED(OLED: Organic Light Emitting Diode) formed on a substrate and sealedbetween the substrate and a cover member, and also to an OLED modulemounting an IC or the like including a controller on such an OLED panel.In the present description, both an OLED panel and an OLED module are,collectively, referred to as a light-emitting device. The presentinvention, furthermore, relates to a driving method to a light-emittingdevice and an electronic apparatus using such a light-emitting device.

2. Description of the Related Art

The OLED, spontaneous to emit light, provides high visibility but doesnot require such a backlight as needed on a liquid-crystal display (LCD)thus optimally reducing the thickness, which is furthermore limitless inviewing angle. Consequently, the light-emitting devices using OLEDs haverecently drawn attentions as the display devices taking the place ofCRTs and LCDs.

The OLED has a layer containing an organic compound (organiclight-emitting material) to cause electroluminescence under theapplication of an electric field (hereinafter, described as organiclight-emitting layer), an anode and a cathode. The electroluminescenceon an organic compound includes the emission of light of upon returningfrom a singlet excitation state into the ground state (fluorescence) andthe emission of light of upon returning from a triplet excitation stateinto the ground state (phosphorescence). The light-emitting device ofthe present invention may use either one or both of such emission oflight.

In the description, every layer provided between an OLED cathode and ananode is defined as an organic light-emitting layer. The organiclight-emitting layer, concretely, includes a light-emitting layer, ahole injection layer, an electron injection layer, a hole transportlayer and an electron transport layer. Basically, the OLED has astructure layered with an anode, a light-emitting layer and a cathode inthe order. In addition to this structure, some structures possess ananode, a hole injection layer, a light-emitting layer and a cathode oran anode, a hole injection layer, a light-emitting layer, an electrontransport layer and a cathode in the order.

It is problematic in placing the light-emitting device into practicalapplication that the brightness of an OLED lowers due to deteriorationin the organic light-emitting material.

The organic light-emitting material is less resistive to moisture,oxygen, light and heat, and to be acceleratingly deteriorated by them.Specifically, the rate of deterioration is dependent upon a devicestructure for driving the light-emitting device, organic light-emittingmaterial properties, electrode materials, conditions in a fabricationprocess, a driving scheme to the light-emitting device and so on.

With even a constant voltage to the organic light-emitting layer, if theorganic light-emitting layer deteriorates, OLED brightness lowers. Thisresults in obscured image display. Note that, in the description, thevoltage applied from a pair of electrodes to an organic light-emittinglayer is defined as an OLED drive voltage (Vel).

Meanwhile, in a color display scheme using three kinds of OLEDscorresponding to R (red), G (green) and B (blue), the organiclight-emitting materials forming the organic light-emitting layer aredifferent between the colors to which the OLEDs correspond.Consequently, there is a possibility that the OLED organiclight-emitting layer deteriorates at a different rate dependently uponthe color. In this case, as time elapses the brightness of OLED becomesdifferent by the color, making impossible for the light-emitting deviceto display an image with a desired color.

Meanwhile, the temperature of the organic light-emitting layer reliesupon the temperature of outside air or the heat generated by the OLEDpanel itself. However, the OLED generally has a flowing current valuevarying with temperature. FIG. 27 shows a change of a voltage-currentcharacteristic of an OLED when changing the temperature of the organiclight-emitting layer. At a constant voltage, when the temperature of theorganic light-emitting layer increases, the OLED drive currentincreases. Because the OLED drive current and the OLED brightness are ina proportional relationship, the brightness on the OLED increases withthe increase in the OLED drive current. In this manner, because OLEDbrightness varies with the temperature of the organic light-emittinglayer, display is difficult at a desired gray scale. The consumptioncurrent of the light-emitting device increases with the rise oftemperature.

Furthermore, because generally the change rate of OLED drive currentagainst temperature change is different depending on the kind of anorganic light-emitting material, there is a possibility in color displaythat the OLED brightness of each color dependently varies withtemperature. The brightness balance, if disordered between the colors,makes impossible to display in a desired color.

It is an object of the present invention to provide a light-emittingdevice capable of obtaining a constant brightness regardless ofdeterioration in an organic light-emitting layer or temperature changeand further of providing display with a desired color.

SUMMARY OF THE INVENTION

The present inventor has paid attentions to the fact that the loweringin OLED brightness due to deterioration is less when light is emittedwhile keeping the current flowing through the OLED at constant ratherthan when light is emitted while keeping the OLED drive voltage atconstant. Note that, in the description, the current flowing through anOLED is referred to as an OLED drive current (lel). It has beenconsidered that the OLED brightness change due to OLED deterioration canbe prevented due to control of the OLED brightness not by a voltage butby a current.

Specifically, the present invention controls the drain current Id of atransistor provided in each pixel by a signal line drive circuit.Because the transistor drain current Id is controlled in the signal linedrive circuit, the drain current Id is placed constant regardless of thevalue of a load resistance.

When a drain current Id flows, a voltage occurs between the gateelectrode and the drain region of the transistor. While maintaining thevoltage the transistor drain current is caused to flow to the OLEDthrough a singular or a plurality of circuit elements. Incidentally, thedrain current Id is in such a magnitude as operating the transistor in asaturation region.

By the above configuration, the OLED drive current flowing through theOLED is controlled in value by the signal line drive circuit regardlessof the value of a load resistance. In other words, the OLED drivecurrent can be controlled to a desired value without being affected bytransistor characteristic difference, OLED deterioration or the like.

The present invention can suppress the lowering in OLED brightness bythe above configuration even where an organic light-emitting layerdeteriorates. As a result, a clear image can be displayed. Meanwhile, inthe case of a color-display light-emitting device using OLEDscorresponding to respective colors, if the OLED organic light-emittinglayer deteriorates at a rate different between the colors prevents thecolors from being unbalanced in brightness, thereby displaying a desiredcolor.

Meanwhile, even where the temperature of the organic light-emittinglayer is influenced by outside-air temperature or the heat generated bythe OLED panel itself, the OLED current can be controlled to a desiredvalue. Accordingly, because the OLED drive current and the OLEDbrightness are in proportion, the OLED brightness can be suppressed fromchanging. Furthermore, consumption current can be prevented fromincreasing due to temperature rise. In the case of a color-displaylight-emitting device, the colors are suppressed from changing in OLEDbrightness without being affected by temperature change. Accordingly,the colors can be prevented from being unbalanced in brightness therebydisplaying a desired color.

Furthermore, because generally the change rate of OLED drive current dueto temperature change is different depending on the kind of an organiclight-emitting material, there is a possibility that the colors in colordisplay variously change in OLED brightness depending on temperature.However, the light-emitting device of the present invention can obtain adesired brightness without being affected by temperature change.Accordingly, the colors are prevented from being unbalanced inbrightness thus displaying a desired color.

Meanwhile, the general light-emitting device has an electric resistanceon the wiring to supply current to each pixel, and hence the potentialthereon somewhat drops due to wiring length. The potential drop largelydiffers depending on an image to be displayed. Particularly, in aplurality of pixels to be supplied by a current from the same wiring,where the ratio of the pixels having many gray scales increases, thecurrent flowing the wiring increases to conspicuously cause potentialdrop. The potential drop decreases the voltage to be applied to the OLEDof each pixel, hence decreasing the current to be supplied to the pixel.Accordingly, in the case to provide display in a constant gray scale ona certain predetermined pixel, when there is change in gray scale onanother pixel being supplied with current through the same wiring, thiscauses a change in the current to be supplied to the predeterminedpixel, resultingly changing the gray scale. However, the light-emittingdevice of the present invention can obtain a measurement value andreference value on each display image thereby correcting the OLEDcurrent. Accordingly, even where there is change in a display image, itcan be displayed at a desired gray scale by the correction.

Incidentally, in the light-emitting device of the present invention, thetransistor for use in the pixel may be a transistor formed usingsingle-crystal silicon or a thin-film transistor using polysilicon oramorphous silicon. Otherwise, the transistor may use an organicsemiconductor.

Incidentally, the transistor provided on the pixel of the light-emittingdevice of the present invention may be of a single-gate structure or amulti-gate structure, such as a double-gate structure or the higher.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram in an upper surface of a light-emitting deviceof the present invention;

FIG. 2 is a circuit diagram of the pixel of the light-emitting device ofthe present invention;

FIGS. 3A and 3B are schematic diagrams of the pixel under driving;

FIG. 4 is a chart showing the occurrence of a write period and displayperiod in an analog driving method;

FIG. 5 is a chart showing the occurrence of a write period and displayperiod in a digital driving method;

FIG. 6 is a schematic diagram of the pixel under driving;

FIG. 7 is a chart showing the occurrence of a write period and displayperiod in a digital driving method;

FIG. 8 is a circuit diagram of the pixel of a light-emitting device ofthe present invention;

FIG. 9 is a circuit diagram of the pixel of a light-emitting device ofthe present invention;

FIG. 10 is a chart showing the occurrence of a write period and displayperiod in the digital driving method;

FIG. 11 is a chart showing the occurrence of a write period and displayperiod in the digital driving method;

FIG. 12 is a chart showing the occurrence of a write period and displayperiod in the digital driving method;

FIG. 13 is a chart showing the occurrence of a write period and displayperiod in the digital driving method;

FIG. 14A to 14C are views showing a fabrication method of alight-emitting device of the present invention;

FIG. 15A to 15C are views showing the fabrication method of alight-emitting device of the present invention;

FIGS. 16A and 16B are views showing the fabrication method of alight-emitting device of the present invention;

FIG. 17 is a top view of the pixel of the light-emitting device of thepresent invention;

FIG. 18 is a sectional view of the pixel of the light-emitting device ofthe present invention;

FIG. 19 is a top view of the pixel of the light-emitting device of thepresent invention;

FIG. 20 is a block diagram of a signal line drive circuit;

FIG. 21 is a detailed diagram of the signal line drive circuit in thedigital driving method;

FIG. 22 is a circuit diagram of a current setting circuit in the digitaldriving method;

FIG. 23 is a block diagram of a first scanning line drive circuit;

FIGS. 24A and 24B are detailed diagrams of the signal line drive circuitin the digital driving method;

FIG. 25A to 25C are an external view and sectional views of thelight-emitting device of the present invention;

FIG. 26A to 26H are views of electronic apparatus using a light-emittingdevice of the present invention; and

FIG. 27 is a figure showing a voltage-current characteristic of theOLED.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment Mode 1

FIG. 1 shows a block diagram of an OLED panel structure of the presentinvention. 100 is a pixel section forming a plurality of pixels 101 in amatrix form. Meanwhile, 102 is a signal line drive circuit, 103 is afirst scanning line drive circuit and 104 is a second scanning linedrive circuit.

In FIG. 1, although the signal line drive circuit 102, the firstscanning line drive circuit 103 and the second scanning line drivecircuit 104 are formed on the same substrate as the pixel section 100,the present invention is not limited to that structure. The signal linedrive circuit 102, the first scanning line drive circuit 103 and thesecond scanning line drive circuit 104 may be formed on a differentsubstrate from the pixel section 100 and connected to the pixel section100 through an FPC or the like. Meanwhile, in FIG. 1, although thesignal line drive circuit 102, the first scanning line drive circuit 103and the second scanning line drive circuit 104 are provided one pereach, the present invention is not limited to that structure. It ispossible for a designer to arbitrarily determine the number of thesignal line drive circuits 102, the first scanning line drive circuits103 and the second scanning line drive circuits 104.

Note that, in the description, connection means electrical connection.

In FIG. 1, on the pixel section 100, there are provided signal linesS1-Sx, power lines V1-Vx, first scanning lines Ga1-Gay and secondscanning lines Gb1-Gby. The signal lines and the power lines are notnecessarily the same in the number. The first scanning lines and thesecond scanning lines are not necessarily the same in the number. Thelight-emitting device of the present invention not necessarily requiresto have all of these lines. Other lines may be provided besides thoselines.

The power lines V1-Vx are held at a predetermined potential. AlthoughFIG. 1 shows a light-emitting device structure to display monochromaticimages, the present invention may be a light-emitting device to displaycolor images. In such a case, the potentials on the power lines V1-Vxmust not be held all in the same height but may be changed based on thecorresponding colors.

FIG. 2 shows a detailed configuration of the pixel 101 shown in FIG. 1.The pixel 101 shown in FIG. 2 has a signal line Si (one of S1-Si), afirst scanning line Gaj (one of Ga1-Gay), a second scanning line Gbj(one of Gb1-Gby) and a power line Vi (one of V1-Vx).

The pixel 101 has a transistor Tr1 (current-control or firsttransistor), a transistor Tr2 (driving or second transistor), atransistor Tr3 (first switching or third transistor), a transistor Tr4(second switching or fourth transistor), an OLED 106 and a storagecapacitor 105.

The sate electrodes of the transistors Tr3 and Tr4 are both connected tothe first scanning line Gaj.

One of the source and drain regions of the transistor Tr3 is connectedto the signal line Si while the other is to the gate electrode of thetransistor Tr1. Meanwhile, one of the source and drain regions of thetransistor Tr4 is connected to the signal line Si while the other is tothe drain region of the transistor Tr1.

The source region of the transistor Tr1 is connected to the power lineVi while the to drain region thereof is connected to the source regionof the transistor Tr2. The gate electrode of the transistor Tr2 isconnected to the second scanning line Gbj. The drain region of thetransistor Tr2 is connected to the pixel electrode possessed by the OLED106.

The OLED 106 has an anode and a cathode. In the description, the cathodeis referred to as a counter electrode (second electrode) where the anodeis used as a pixel electrode (first electrode) and the anode is referredto as a counter electrode where the cathode is used as a pixelelectrode.

The potential oh the counter electrode is kept at a constant height.

Note that the transistor Tr3 and the transistor Tr4 may be n-channeltransistors or p-channel transistors. However, the transistor Tr3 andthe transistor Tr4 have the same to polarity.

Meanwhile, the transistors Tr1 and Tr2 each may be either an n-channeltransistor or a p-channel transistor. However, the transistors Tr1 andTr2 have the same polarity. In the case of using the anode as a pixelelectrode and the cathode as a counter electrode, the transistors Tr1and Tr2 are p-channel transistors. Conversely, where the anode is usedas a counter electrode and the cathode as a pixel electrode, thetransistors Tr1 and Tr2 are n-channel transistors.

The storage capacitor 105 is formed between the gate electrode of thetransistor Tr1 and the power line Vi. The storage capacitor 105,provided to maintain a voltage between the gate electrode and the sourceregion of the transistor Tr1 (gate voltage), is not necessarilyprovided.

Embodiment Mode 2

Now, explanation will be made on the drive to the light-emitting deviceshown in FIG. 2, using FIG. 3. In the present embodiment mode, theoperation of each pixel of the light-emitting device shown in FIG. 2 isexplained by separating with a write period Ta and a display period Td.

In the write period Ta, the first scanning line Gaj is selected. Whenthe first canning line Gaj is selected, the transistors Tr3 and Tr4connected at their gate electrode to the first scanning line Gaj turnon. Note that, in the write period Ta, the second scanning line Gbj isnot selected and hence the Tr2 is off.

On the basis of a potential of a video signal inputted to the signalline drive circuit 102, constant currents Ic flow respectively betweenthe signal lines S1-Sx and the power lines V1-Vx. Note that, in thedescription, the current Ic is referred to as a signal current.

FIG. 3A shows a schematic diagram of the pixel 101 that, in the writeperiod Ta, a constant current Ic flows on the signal line Si. 107 meansa constant-current source possessed by the signal line drive circuit102. Meanwhile, 108 is a connection terminal to a power source forgiving a potential to the counter electrode.

In the write period Ta, the transistors Tr3 and Tr4 are in an on state.Accordingly, when a constant current Ic flows to the signal line Si, theconstant current Ic flows between the source and drain regions of thetransistor Tr1. At this time, the current Ic is controlled in itsmagnitude by the constant current source 107 so as to operate thetransistor Tr1 in a saturation region.

In the saturation region, provided that V_(GS) is a potential differencebetween the gate electrode and the source region (gate voltage), μ is atransistor mobility. C₀ is a gate capacitance per unit area, W/L is aratio of a channel width W to a channel length L in the channel region.V_(TH) is a threshold value, μ is a mobility and Id is a drain currentof the transistor Tr1, the following Equation 1 is held.Id=μC ₀ W/L(V _(GS) −V _(TH))²/2  Equation 1

In Equation 1, μ, C₀, W/L and V_(TH) are fixed values as determined bythe individual transistor. Meanwhile, the drain current Id of thetransistor Tr1 is held Ic=Id by the constant current source 107.Accordingly, as can be seen from Equation 1, the gate voltage V_(GS) ofthe transistor Tr1 is determined by a value of signal current Ic.

When the write period Ta terminates, a display period Td commences. Inthe display period Td, the first scanning line Gaj is not selected butthe second scanning line Gbj is selected.

FIG. 3B shows a schematic diagram of the pixel in the display period Td.The transistor Tr3 and the transistor Tr4 are off. Meanwhile, thetransistor Tr2 is on.

In the display period Td, the transistor Tr1 is maintained with V_(GS),as it is, determined in the write period Ta. Consequently, the draincurrent Id in value of the transistor Tr1 remains at the same value asthe signal current Ic. Meanwhile, because the transistor Tr2 is on, thedrain current Id flows to the OLED 106 through the transistor Tr2.Consequently, in the display period Td, an OLED drive current having thesame magnitude as the signal current Ic flows to the OLED 106 and theOLED 106 emits light at a brightness commensurate with the magnitude ofthe OLED drive current.

Write period Ta and display period Td occur on all the pixels. Thetiming of occurrence differs from pixel to pixel on each line. Notethat, in the description, all the pixels having the same first scanningline or the same second scanning line of among a plurality of pixelspossessed by the pixel section are referred to as the pixels on the sameline.

In the case with a driving method using an analog video signal (analogdriving method), the magnitude of Ic is determined by the analog videosignal to cause the OLED 106 to emit light at a brightness commensuratewith the magnitude of Ic, thereby providing tonal representation. Inthis case, one image is displayed by the occurrence of one write periodTa and one display period Td on all the pixels. The duration of from acommencement of write period Ta on any one pixel to a termination ofdisplay period Td on all the pixels is referred to as a frame period.The successive frame periods are overlapped one with another.

FIG. 4 shows one example of a timing chart in the analog driving method.One frame period has line periods in the number of y, wherein each firstscanning line is selected in each line period. In the line period, apredetermined signal current Ic (Ic1-Icx) flows on each signal line. InFIG. 4, the value of a signal current flowing on each signal line in theline period Lj (j=1−y) is represented as Ic1[Lj]−Icx[Lj].

The start timing of the write period Ta and display period Td deviatesfrom pixel to pixel on each line so that there is no overlap in thetiming of write period occurrence between the pixels on each line.

On the other hand, in the case with a time-gray scale driving methodusing a digital video signal (digital driving method), a write period Taand display period Td repeatedly occurs during one frame period on eachpixel, thereby making possible to display one image. Where an image isdisplayed by an n-bit video signal, at least write periods in the numberof n and display periods in the number of n corresponding to each bitare provided in one frame period. The write periods (Ta1-Tan) in thenumber of n and the display periods (Td1-Tdn) in the number of ncorrespond to each bit of the video signal.

FIG. 5 shows the timing that write periods (Ta1-Tan) in the number of nand display periods (Td1-Tdn) in the number of n occur in one frameperiod. The horizontal axis represents a time while the vertical axisrepresents a position of the first scanning line possessed by the pixel.

Next to the write period Tam (m: arbitrary number of 1-n), a displayperiod occurs that corresponds to the same bit number, i.e. Tdm in thiscase. The write period Ta and the display period Td are, collectively,referred to as a sub-frame time SF. The sub-frame period having a writeperiod Tam and display period Tdm corresponding to m-th bit is givenSFm.

The length of a display time Td1-Tdn satisfies Td1:Td2: . . .:Tdn=2⁰:2¹: . . . :2^(n−1).

Note that, in order to improve the image quality on display, thesub-frame period long in display duration may be divided into a certainsegments. How to concretely divide is disclosed in Japanese PatentApplication No. 2000-267164 which can be referred. In the driving methodshown in FIG. 5, gray scale is controlled by controlling the sum inlength of the display periods of light emission within one frame period.

In the present invention, the above configuration can suppress thelowering in OLED brightness even where the organic light-emission layeris deteriorated. As a result, a clear image can be displayed. Meanwhile,in the case of a color-display light-emitting device using OLEDscorresponding to respective colors, even where the OLED light-emittinglayers deteriorate at different rate depending on the correspondingcolors, the colors are prevented from being unbalanced in brightnessthereby enabling to display a desired color.

Meanwhile, even if the temperature of the organic light-emitting layerbe affected by outside-air temperature or the heat caused due to theOLED panel itself, the OLED drive current can be controlled to a desiredvalue. Consequently, because the OLED drive current and the OLEDbrightness are in proportion, the OLED brightness can be prevented fromchanging. Also, consumption current can be prevented from increasingwith rise in the temperature. In the case of a color-displaylight-emitting device, the OLED brightness on each color can besuppressed from changing irrespectively of temperature change.Accordingly, the colors can be prevented from being unbalanced inbrightness, enabling display with a desired color.

Furthermore, because generally the change rate of OLED drive current dueto temperature change is different depending on the kind of an organiclight-emitting to material, there is a possible case that the OLEDbrightness on each color randomly varies with temperature. However, inthe light-emitting device of the present invention, because a desiredbrightness can be obtained without being affected by temperature change,the colors can be prevented from being unbalanced in brightness therebyenabling to display a desired color.

Meanwhile, because the general light-emitting device has electricresistance in the wiring itself for supplying current to each pixel,somewhat potential drop occur due to the length of the wiring. Thepotential drop largely differs depending also upon an image to bedisplayed. Particularly, on a plurality of pixels to be supplied withcurrent through the same wiring, when the ratio of the pixel having manygray scales increases, the current flowing on the wiring increases tocause potential drop conspicuously. When the potential drops, thevoltage applied to OLED of the pixel decreases to decrease the currentto be supplied to the pixel. Accordingly, in the case to display at acertain gray scale on a certain predetermined pixel, when there ischange in gray scale on another pixel being supplied with currentthrough the same wiring, the current being supplied to the predeterminedpixel changes to resultingly change the gray scale. However, thelight-emitting device of the present invention can obtain a measurementvalue and reference value on each pixel to be displayed, therebycorrecting the OLED current. It is therefore possible to display at adesired gray scale even if there is change in display image.

Embodiment Mode 3

This embodiment mode explains an embodiment mode different fromEmbodiment Mode 2 of driving the light-emitting device of FIG. 2, usingFIG. 6. In this embodiment mode, the operation of each pixel of thelight-emitting device of FIG. 2 will be explained by separating with awrite period Ta, a display period Td and a non-display period Te. Notethat, because the operation on the pixel in the write period Ta anddisplay period Td has already been explained in Embodiment Mode 2,explanation will be herein made on the operation on the pixel in anon-display period Te.

The non-display period Te occurs after terminating a display period Tdand before an occurrence of a display period Td. In the non-displayperiod Td, the first scanning line Gaj and the second scanning line Gbjare not selected.

FIG. 6 shows a schematic diagram of the pixel in a non-display periodTe. The transistor Tr3 and the transistor Tr4 are off. Meanwhile, thetransistor Tr2 is also off. Accordingly, no OLED drive current flows tothe OLED 106, and hence the OLED 106 will not emit light.

A non-display period Te not always occurs following the display periodTd. However, in such a case that the display period terminates on thepixels on the first line before the write period terminates on all thelines, a non-display period occurs after the display period.

The driving method of this embodiment mode is used mainly in the drivewith a digital video signal. In the driving method for time gray scaleusing a digital video signal (digital driving method), a write period Taand a display period Td repeatedly occur on each pixel in one frameperiod, thereby enabling to display one image. In the case of displayingan image by an n-bit video signal, at least write periods in the numberof n and display periods in the number of n are provided in one frameperiod. The write periods in the number of n (Ta1-Tan) and displayperiods in the number of n (Td1-Tdn) correspond to each bit of a videosignal.

FIG. 7 shows the occurrence timing of write periods in the number of n(Ta1-Tan), display periods in the number of n (Td1-Tdn) and onenon-display period (Tel-Tel) within one frame period. Note that thisembodiment mode explains on the case of 1=n−3 in order to simplifyexplanation. The horizontal axis represents a time while the verticalaxis represents a position of the first scanning line possessed by thepixel. Meanwhile, because write period is short, the commencement timingof a write period Ta1-Tan corresponding to each bit is denoted at anarrow in order for easily seeing the figure. Meanwhile, the period, foreach bit, of from a commencement of a write period to the pixel on thefirst line to a termination of a write period to the pixel on a y-thline is shown with ΣTa1-ΣTan.

In a write period Ta1, the drain current of the transistor Tr1 iscontrolled by a first-bit digital video signal in the order of from thepixel on the first line. Next, when a display period Td1 commences, thetransistors Tr3 and Tr4 turn off and the transistor Tr2 turns on in theorder of from the pixel on the first line, thereby flowing a draincurrent through the OLED 106. Consequently, the OLED 106 is placed in alight emission or non-emission state.

Then, a non-display period Te1 commences so that the transistor Tr2turns into an off state while the transistors Tr3 and Tr4 remain off inthe order of from the pixel on the first line. Accordingly, the draincurrent becomes not to flow through the OLED 106 and hence the OLED 106goes into an off state.

Then, a write period Ta2 commences to repeat the foregoing operationuntil the non-display period Te(n−3) terminates.

When the non-display period Te(n−3) terminates, a write period Ta(n−2)commences whereby the drain current of the transistor Tr1 is controlledby a (n−2)-th bit digital video signal in the order of from the pixel onthe first line. When a display period Td(n−2) then commences, thetransistors Tr3 and Tr4 turn off and the transistor Tr2 turns on in theorder of from the pixel on the first line, thereby flowing a draincurrent through the OLED 106. Consequently, the OLED 106 is placed in astate of light emission or non-emission.

Then, a write period Ta(n−1) commences to repeat the foregoing operationuntil the display period Tdn terminates.

After Tdn terminates on the pixels of the first line, one frame periodcompletes. Again a write period Ta1 in the next frame period commenceson the pixel of the first line. Thus, the foregoing operation is againrepeated. The commencement timing and termination timing of one frameperiod has time difference between the pixels of each line.

When the display period Tdn terminates on all the pixels, one image canbe displayed.

Incidentally, it is assumed that the display period length isTd1:Td2:Td3: . . . Td(n−1):Tdn=2⁰:2¹:2²: . . . 2^((n−2)):2^((n−1)). Thecombination of such display periods provides a representation at adesired gray scale of among 2^(n) gray scales.

EMBODIMENT

Explanations will be made below on the examples of the presentinvention.

Embodiment 1

This embodiment explains a pixel configuration different from that ofthe light-emitting device of the present invention of FIG. 2.

The OLED panel possessed by the light-emitting device of the presentinvention does not have a second scanning line drive circuit differentlyfrom the OLED panel shown in FIG. 1. Note that, in this embodiment, thefirst scanning line drive circuit is referred simply to as a scanningline drive circuit.

The OLED panel of this embodiment has a pixel section forming aplurality of pixels in a matrix form, a signal line drive circuit and ascanning line drive circuit.

The signal line drive circuit and the scanning line drive circuit may beformed on the same substrate as the pixel section. Otherwise, they maybe formed on respective different substrates and connected to the pixelsection through FPCs or the like. Meanwhile, it is possible for adesigner to arbitrarily determine the number of signal line drivecircuits and scanning line drive circuits.

In the pixel section, there are provided signal lines S1-Sx, power linesV1-Vx and scanning lines G1-Gy. Note that the signal lines and the powerlines are not necessarily the same in the number. The light-emittingdevice of the present invention not necessarily requires all of theselines, and other wirings than those may be provided.

The power lines V1-Vx are held at a predetermined potential. The powerlines V1-Vx, in all, do not require the same height of potential.

FIG. 8 shows a concrete configuration of a pixel in this embodiment. Thepixel 201 shown in FIG. 8 has a signal line Si (one of S1-Sx), ascanning line Gj (one of G1-Gy) and a power line Vi (one of V1-Vx).

The pixel 201 has also a transistor Tr1 (current control or firsttransistor), a transistor Tr2 (driving or second transistor), atransistor Tr3 (first switching or third transistor), a transistor Tr4(second switching or fourth transistor), an OLED 206 and a storagecapacitor 205.

The gate electrodes of the transistors Tr3 and Tr4 are both connected tothe scanning line Gj.

One of the source and drain regions of the transistor Tr3 is connectedto the signal line Si and the other is connected to the gate electrodeof the transistor Tr1. Meanwhile, one of the source and drain regions ofthe transistor Tr4 is connected to the signal line Si and the other isconnected to the drain region of the transistor Tr1.

The source region of the transistor Tr1 is connected to the power lineVi and the drain region thereof is connected to the source region of thetransistor Tr2. The gate electrode of the transistor Tr2 is connected tothe scanning line Gj. The drain region of the transistor Tr2 isconnected to the pixel electrode possessed by the OLED 206.

The OLED 206 has an anode and a cathode.

The potential on the counter electrode is held at a constant height.

Note that the transistors Tr3 and Tr4 may be n-channel transistors orp-channel transistors. However, the transistor Tr3 and the transistorTr4 are the same in polarity.

Meanwhile, the transistors Tr1 and Tr2 have a polarity reverse to thatof the transistors Tr3 and Tr4. Consequently, when the transistors Tr3and Tr4 are on, the transistor Tr2 is off. Conversely, when thetransistors Tr3 and Tr4 are off, Tr2 is on.

In the case of using the anode as a pixel electrode and the cathode as acounter electrode, the transistors Tr1 and Tr2 are p-channeltransistors. Conversely, where using the anode as a counter electrodeand the cathode as a pixel electrode, the transistors Tr1 and Tr2 aren-channel transistors.

The storage capacitor 205 is formed between the gate electrode of thetransistor Tr1 and the power line Vi. The storage capacitor 205 may notnecessarily be provided although provided in order to maintain a voltageof between the gate electrode and the source region of the transistorTr1 (gate voltage).

The pixel shown in FIG. 8 operates by the driving method shown inEmbodiment Mode 2. Namely, as shown in FIG. 3, the operation can beexplained by separation with a write period and a display period.Incidentally, the detailed operating manner on the pixel can be referredto Embodiment Mode 2, and hence is omitted herein.

Embodiment 2

This embodiment explains a configuration of a pixel of thelight-emitting device of the present invention different from that ofFIG. 2 or 8.

The OLED panel possessed by the light-emitting device of this embodimenthas a pixel section forming a plurality of pixels in a matrix form, asignal line drive circuit, a first scanning line drive circuit and asecond scanning line drive circuit, similarly to the OLED panel shown inFIG. 1.

The signal line drive circuit, the first scanning line drive circuit andthe second scanning line drive circuit may be formed on the samesubstrate as the pixel section. Otherwise, they may be formedrespectively on different substrates and connected to the pixel sectionthrough FPCs or the like. Meanwhile, it is possible for a designer toarbitrarily determine the number of signal line drive circuits, firstscanning line drive circuits and second scanning line drive circuits.

In the pixel section, there are provided signal lines S1-Sx, power linesV1-Vx, first scanning lines Ga1-Gay and second scanning lines Gb1-Gby.Note that the signal lines and the power lines are not necessarily thesame in the number. Meanwhile, the first scanning lines and the secondscanning lines are not necessarily the same in the number. Thelight-emitting device of the present invention not necessarily requiresto have all of these lines. Other wirings may be provided besides thosewirings.

The power lines V1-Vx are held at a predetermined potential. The powerlines V1-Vx in all do not require the same height of potential.

FIG. 9 shows a concrete configuration of a pixel in this example. Thepixel 211 shown in FIG. 9 has a signal line Si (one of S1-Sx), a firstscanning line Gaj (one of Ga1-Gay), a second scanning line Gbj (one ofGb1-Gby) and a power line Vi (one of V1-Vx).

The pixel 211 also has a transistor Tr1 (current control or firsttransistor), a transistor Tr2 (driving or second transistor), atransistor Tr3 (first switching or third transistor), a transistor Tr4(second switching or fourth transistor), a transistor Tr5 (erasing orfifth transistor), an OLED 216 and a storage capacitor 215.

The gate electrodes of the transistors Tr3 and Tr4 are both connected tothe first scanning line Gaj.

One of the source and drain regions of the transistor Tr3 is connectedto the signal line Si while the other is connected to the gate electrodeof the transistor Tr1. Meanwhile, one of the source and drain regions ofthe transistor Tr4 is connected to the signal line Si while the other isconnected to the drain region of the transistor Tr1.

The source region of the transistor Tr1 is connected to the power lineVi and the drain region thereof is connected to the source region of thetransistor Tr2. The gate electrode of the transistor Tr2 is connected tothe first scanning line Gaj. The drain region of the transistor Tr2 isconnected to the pixel electrode possessed by the OLED 216.

The gate electrode of the transistor Tr5 is connected to the secondscanning line Gbj. One of the source and drain regions of the transistorTr5 is connected to the power line Vi while the other is connected tothe gate electrode of the transistor Tr1.

The OLED 206 has an anode and a cathode.

The potential on the counter electrode is held at a constant height.

Note that the transistors Tr3 and Tr4 may be n-channel transistors orp-channel transistors. However, the transistor Tr3 and the transistorTr4 are the same in polarity.

Meanwhile, the transistors Tr1 and Tr2 have a polarity reverse to thatof the transistors Tr3 and Tr4. Consequently, when the transistors Tr3and Tr4 are on, the transistor Tr2 is off. Conversely, when thetransistors Tr3 and Tr4 are off, Tr2 is on.

In the case of using the anode as a pixel electrode and the cathode as acounter electrode, the transistors Tr1 and Tr2 are p-channeltransistors. Conversely, where using the anode as a counter electrodeand the cathode as a pixel electrode, the transistors Tr1 and Tr2 aren-channel transistors.

The storage capacitor 215 is formed between the gate electrode of thetransistor Tr1 and the power line Vi. The storage capacitor 215 may notnecessarily be provided although provided in order to maintain a voltageof between the gate electrode and the source region of the transistorTr1 (gate voltage).

The pixel shown in FIG. 9 operates on the driving method shown inEmbodiment Mode 3. However, in the case of the pixel shown in FIG. 9,the operation on the pixels in a non-display period is different fromthat shown in FIG. 6. In the pixel of FIG. 9, by turning on thetransistor Tr5 in the non-display period, the gate voltage Tr1 goes nearto 0, and hence Tr1 turns off. Because the transistor Tr2 is on but Tr1is off, an OLED drive current does not flow through the OLED 216 andhence the OLED 216 turns off. Consequently, it is possible to explainthe operation by separation with a write period, a display period and anon-display period. Incidentally, concrete drive timing can be referredto Embodiment Mode 3 and hence is omitted herein.

Embodiment 3

This example describes an order sub-frame periods SF1 to SFn turn up ina driving method shown in Embodiment Mode 2.

FIG. 10 shows at which points n writing periods (Ta1 to Tan) and ndisplay periods (Td1 to Tdn) are started in one frame period. Thehorizontal axis indicates time whereas the vertical axis indicatespositions of the first scanning lines of pixels. Descriptions on detailsabout how the pixels operate are omitted here but can be found inEmbodiment Mode 2.

According to the driving method of this embodiment, the sub-frame periodthat has the longest display period in one frame period (in thisexample, SFn) does not come first or last in the one frame period. Inother words, the sub-frame period that has the longest display period inone frame period is interposed between other sub-frame periods of thesame frame period.

This makes it difficult for the human eye to recognize uneven displaycaused by light emission in close display periods in adjacent frameperiods when an image is displayed with intermediate gradations.

The structure of this example is effective when n≧3. This embodiment maybe combined freely with Embodiment Mode 1.

Embodiment 4

This example describes a driving method which is different from thoseillustrated in Embodiment 3.

FIG. 11 shows at which points n+1 writing periods (Ta1 to Ta(n+1)) andn+1 display periods (Td1 to Td(n+1)) are started in one frame period.The horizontal axis indicates time whereas the vertical axis indicatespositions of the first scanning lines of pixels. Descriptions on detailsabout how the pixels operate are omitted here but can be found inEmbodiment Mode 2.

In this embodiment, one frame period has n+1 sub-frame periods SF1 toSF(n+1) in accordance with n bit digital video signals. The sub-frameperiods SF1 to SF(n+1) have n+1 writing periods (Ta1 to Ta(n+1)) and n+1display periods (Td1 to Td(n+1)).

A writing period Tam (m is an arbitrary number ranging from 1 to n+1)and a display period Tdm make a sub-frame period SFm. The writing periodTam is followed by a display period associated with the same bit number,in this case, the display period Tdm.

The sub-frame periods SF1 to SF(n−1) are respectively associated with 1bit digital video signals to (n−1) bit digital video signals. Thesub-frame periods SFn and SF(n+1) are both associated with the n-th bitdigital video signals.

The sub-frame periods SFn and SF(n+1) that are for digital video signalsof the same bit number do not immediately follow each other in thisexample. In other words, the sub-frame periods SFn and SF(n+1) that arefor digital video signals of the same bit number sandwich anothersub-frame period.

A writing period Ta and a display period Td are repeatedly alternated inone frame period to make it possible to display one image.

Lengths of the display periods Td1 to Td(n+1) are set so as to satisfyTd1:Td2: . . . :(Tdn+Td(n+1))=2⁰:2¹: . . . :2^(n−1).

According to the driving method of the present invention, gradationdisplay is obtained by controlling the total light emission time of apixel in one frame period, namely, for how many display periods in oneframe period the pixel emits light.

The above structure makes the uneven display in intermediate gradationdisplay less recognizable to the human eye than in the cases illustratedin FIGS. 5 and 10. The uneven display is caused by adjoining displayperiods during which light is emitted in adjacent frame periods.

Described in this embodiment is the case in which two sub-frame periodsare provided for digital video signal of the same bit. However, thepresent invention is not limited thereto. Three or more sub-frameperiods may be provided for the same bit in one frame period.

Although a plurality of sub-frame periods are provided for the mostsignificant bit digital video signal in this embodiment, the presentinvention is not limited thereto. A digital video signal of other bitthan the most significant bit may have a plurality of sub-frame periods.There is no need to limit the number of digital video signal bits thatcan have a plurality of sub-frame periods to one. A digital video signalof certain bit and a digital video signal of another bit canrespectively have plural sub-frame periods.

The structure of this embodiment is effective when n≧2. This example canbe combined freely with Embodiments 1 and 3.

Embodiment 5

This embodiment explains the sequence of occurrence in the drivingmethod shown in Embodiment Mode 3. Note that this embodiment explainsthe case with n=6 and l=5. Incidentally, the example explains mere oneexample of a driving method of the present invention wherein the presentinvention is not limited to the configuration of this embodiment inrespect of the bit number n or 1 of a corresponding digital videosignal. The configuration of the embodiment is effective for the casethat the digital video signal has bits of 3 or more.

FIG. 12 shows the occurrence timing of a write period, a display periodor a non-display period in the driving method in this embodiment. Thehorizontal axis represents a time while the vertical axis represents aposition of the first scanning line and second scanning line possessedby the pixel. Note that, because the write period is short, the timing awrite period Ta1-Ta6 corresponding to each bit commences is denoted atan arrow in order for easily seeing the figure. Meanwhile, the period(ΣTa1-ΣTa6) for each bit is shown at an arrow, which is of from acommencement of a write period to the pixel on the fist line to atermination of a write period to the pixel on a y-th line.

The concrete operation on the pixel can be referred to Embodiment Mode 3and hence is omittedly explained herein.

At first, a write period Ta4 commences on the pixel in the first line.When the write period Ta4 commences, a fourth-bit digital video signalis inputted to the pixel of the first line.

After the write period Ta4 terminates on the pixels of the first line,then a write period Ta4 commences sequentially on the pixels in thesecond line or the following.

Similarly to the pixels of the first line, fourth-bit digital videosignal is inputted to the pixels of each line.

On the other hand, concurrently with the commencement of a write periodTa4 on the pixel of the second line or the following, a display periodTd4 commences on the pixel of the first line. When the display periodTd4 commences, display is made by the pixels of the first line accordingto the fourth-bit digital video signal.

Then, after commencing the display period Td4 on the pixels of the firstline, the write period Ta4 terminates sequentially on the pixels of thesecond line or the following thereby commencing a display time Td4.Display is made on the pixels of each line according to a fourth-bitdigital video signal.

On the other hand, after a display time Td4 commences on the pixel ofthe second line or the following, the display period Td4 terminates onthe pixels of the first line to commence a non-display period Te4.Incidentally, concurrently with the commencement of a display period Td4on the pixels of the second line or the following, the display periodTd4 may terminate on the pixels of the first line to commence anon-display period Te4.

By commencing a non-display period Te4, the pixels of the first linebecome not to make display.

Next, after a non-display period Te4 commences on the pixel of the firstline, the display period Td4 terminates also on the pixel of the secondline or the following to commence a non-display period Te4.Consequently, the pixels on each line become not to make display.

On the other hand, in concurrent with the commencement of a non-displayperiod Te4 on the pixel of the second line or the following or aftercommencing a non-display period Te4 on all the pixels, a write periodTa5 commences on the pixel of the first line.

When a write period Ta5 commences on the pixel of the first line, afifth-bit digital video signal is inputted to the pixels of the firstline. When the write period Ta5 terminates on the pixels of the firstline, a write period Ta5 commences sequentially on the pixels of thesecond line or the following.

On the other hand, after terminating the write period Ta5 on the pixelsof the first line, a display period Td5 commences on the pixel of thefirst line concurrently with the commencement of a write period Ta5 onthe pixel of the second line or the following. In the display periodTd5, the pixels make display according to a fifth-bit digital videosignal similarly to that in the display period Td5.

After a display period Td5 commences on the pixel of the first line, thewrite period Ta5 terminates sequentially on the pixels of the secondline or the following to commence a display period Td5.

Next, after a display period Td5 has commenced on the pixel of all thelines, the display period Td5 terminates on the pixels of the first lineto commence a write period Ta2.

When a write period Ta2 commences on the pixel of the first line, asecond-bit digital video signal is inputted to the pixels of the firstline.

When the write period Ta2 terminates on the pixels of the first line, awrite period Ta2 commences sequentially on the pixels of the second lineor the following. Similarly to the case of the pixels on the first line,a second-bit digital video signal is inputted to the pixels of eachline.

On the other hand, concurrently with the commencement of a write periodTa2 on the pixel of the second line or the following, a display periodTd2 commences on the pixel of the first line. When a display period Td2commences, the pixels of the first line make display according to asecond-bit digital video signal.

After commencing a display period Td2 on the pixels of the first line,the write period Ta2 terminates sequentially on the pixels of the secondline or the following to commence a display period Td2. The pixels ofeach line make display according to a second-bit digital video signal.

On the other hand, concurrently with the commencement of a display timeTd2 on the pixel of the second line or the following, the display periodTd2 terminates on the pixels of the first line to commence a non-displayperiod Te2.

By commencing a non-display period Te2, the pixels of first line becomenot to make display.

Next, after commencing a non-display period Te2 on the pixel of thefirst line, the display period Td2 terminates sequentially on the pixelsof the second line or the following to commence a non-display periodTe2. Consequently, the pixels of each line become not to display.

On the other hand, concurrently with the commencement of a non-displayperiod Te2 on the pixel of the second line or the following or afterhaving commenced a non-display period Te2 on all the pixels, a writeperiod Ta3 commences on the pixel of the first line.

The above operation is repeated before all the first- to sixth-bitdigital video signals have been inputted to the pixels, whereby a writeperiod Ta, a display period Td and a non-display period Te occurrepeatedly on a pixel-by-pixel basis of each line.

After terminating all the display periods Td1-Td6 on the pixels of thefirst line, one frame period completes on the pixels of the first lineto again commence a first write period of the next frame period (T4 inthis embodiment). Meanwhile, after one frame period completes on thepixels of the first line, one frame period completes also on the pixelsof the second line or the following to again commence a write period Ta4of the next frame period.

The foregoing operation is repeated again. The commencement andterminating timing of one frame period has a time difference on apixel-by-pixel basis of each line.

By terminating one frame period on the pixels of all the lines, oneimage can be displayed.

In this embodiment, the display periods assumably has a length ofTd1:Td2: . . . :Td5:Td6=2⁰:2¹: . . . :2⁴:2⁵. The combination of thedisplay periods allow for representation at a desired gray scale ofamong 2⁶ gray scales.

By determining a sum in length of the display period for which the OLEDhas emitted light within one frame period, fixed is a gray scale thatthe pixel has made display within the same frame period. For example, inthe case of the present embodiment, provided that the brightness thatthe pixel emits light for the total display period is 100%, the lightemission on the pixel for Td1 and Td2 can give a representation at abrightness of 5%. Where selecting Td3 and Td5, it is possible to give arepresentation at a brightness of 32%.

Incidentally, the write period does not overlap between the pixels oneach line, after terminating the write period on the pixels of a y-thline, a write period is commenced on the pixel of the first line.

In this embodiment, it is essential that the display period Td5 on thepixels of each line is longer than the period (ΣTa5) of from acommencement of a write period Ta5 on the pixel of the first line to atermination of a write period Ta5 on the pixels of the y-th line.

The display periods Td1-Td6 may occur in any of sequence. For example,it is possible for display periods to occur in the sequence of, next toTd1, Td3, Td5, Td2, . . . . within one frame period. However, the writeperiods must not be overlapped between the pixels on each line.

The driving method of the present invention can make the display periodon the pixel of each line shorter than the duration of from acommencement of a write period Ta on the pixels of the first line to atermination of a write period on the pixel of the y-th line, in otherwords shorter than the duration to write 1-bit digital video signal toall the pixels. Accordingly, even where the digital video signalincreases in bits, the display period corresponding to the lower bit canbe made short. Thus, it is possible to display a precise image withoutcausing flicker on the screen.

Meanwhile, the light-emitting device of the present invention can obtaina constant brightness without being affected by temperature change. Evenwhere an OLED having EL materials different between colors is providedin color display, it is possible to prevent against the random variationin brightness on the OLEDs between the respective colors due totemperature, i.e. against the impossibility to obtain a desired color.

Note that, in the driving method of this embodiment, the longest displayperiod within one frame period (Td6 in this embodiment) is not providedat the beginning or end of the one frame period. In other words,configuration is made such that, prior to or after the longest displayperiod within one frame period, another display period included in thesame frame period occurs.

The above configuration makes it possible for the human eye to lessrecognize the unevenness of display caused by the adjacent displayperiods to emit light at between the adjacent frame periods duringdisplay with a neutral gray scale.

The structure of this example can be combined freely with Embodiment 2.

Embodiment 6

This example gives a description on an embodiment of a driving method,which is different from the one described in Example 5, and uses n bitdigital video signals. The case described in this embodiment is aboutwhen 1=n−2.

The driving method in this example has the display period Tdn and thedisplay period Td(n+1) that are associated with the most significant bitdigital video signal. The writing period Tan and the writing periodTa(n+1) are provided, which are associated with the display period Tdnand the display period Td(n+1) respectively.

In this embodiment, the display period Tdn and the display periodTd(n+1) that are associated with same bit of the digital video signal donot appear in succession. In other words, another display period isprovided between the display period Tdn and Td(n+1) that are associatedwith same bit of the digital video signal.

FIG. 13 is a timing diagram of writing periods, display periods, andnon-display periods according to the driving method of this embodiment.The horizontal axis indicates time and the vertical axis indicates theposition of the first scanning line and the second scanning line. Thewriting periods are not shown as bands in FIG. 13 because they areshort. Instead, for less crowded view, arrows indicate starting pointsof the writing periods Ta1 to Ta(n+1). A period that begins with thestart of a writing period in the pixels on first line and ends with theend of a writing period in the pixels on y-th line for a 1 bit digitalvideo signal is denoted by Ta1 to ΣTa(n+1) and indicated by an arrow.

Details about how pixels operate are described in Embodiment Modes andthe explanation is therefore omitted here.

The length of the display periods Td1 to Td(n+1) is set so as to satisfyTd1:Td2:. . . :Td(n−1):(Tdn+Td(n+1))=2⁰:2¹: . . . :2⁻²:2^(n−1).

The gradation display is obtained by controlling the total lightemission time of a pixel in one frame period.

The above structure makes the uneven display in middle gradation displayless recognizable to the human eye than in Embodiment 2. The unevendisplay is caused by adjoining display periods during which light isemitted from pixels in adjacent frame periods.

Described in this embodiment is the case in which two display periodsare provided for the digital video signal of the same bit. However, thepresent invention is not limited thereto. Three or more display periodsmay be provided for the digital video signal of the same bit in oneframe period.

Although a plurality of display periods are provided for the mostsignificant bit digital video signal, the present invention is notlimited thereto. A digital video signal of other bit than the mostsignificant bit may have a plurality of display periods. There is noneed to limit the number of digital video signal bits that can have aplurality of display periods to one. A digital video signal of certainbit and a digital video signal of another bit can respectively haveplural display periods.

The structure of this embodiment is effective when n≧2. This example canbe combined freely with Embodiment s 2 or 5.

Embodiment 7

This embodiment gives a description on a method of manufacturing alight-emitting device according to the present invention. In thisembodiment, a description uses the pixel manufacturing method in FIG. 2.Though the cross sectional diagram of transistors Tr2 and Tr4 of pixelsare only shown in this embodiment, the transistors Tr1 and Tr3 can beformed referring to the manufacturing method of this embodiment. Anothertransistors of pixels (for example, the transistor Tr5 as for pixelsshown in FIG. 9) can be formed by the same method. In addition, drivingcircuits (a signal line driving circuit, a first scanning line drivingcircuit, and a second scanning line driving circuit) provided in theperiphery of the pixel portion may be formed on the same substrate onwhich the TFTs for the pixel portion are placed at the same time thepixel portion TFTs are formed.

First, as shown in FIG. 14A, a base film 302 is formed from aninsulating film such as a silicon oxide film, a silicon nitride film,and a silicon oxynitride film on a glass substrate 301. The substrate301 is formed of barium borosilicate glass typical example of which isCorning # 7059 glass or Corning #1737 glass (product of CorningIncorporated), or of aluminoborosilicate glass. The base film 302 is,for example, a laminate of a silicon oxynitride film 302 a that isformed from SiH₄, NH₃, and N₂O by plasma CVD to a thickness of 10 to 200nm (preferably 50 to 100 nm) and a silicon oxynitride hydride film 302 bformed from SiH₄ and N₂O by plasma CVD to a thickness of 50 to 200 nm(preferably 100 to 150 nm). Although the base film 302 in this examplehas a two-layer structure, it may be a single layer of one of theinsulating films given in the above, or a laminate of two or more layersof those insulating films.

A semiconductor film having an amorphous structure is crystallized bylaser crystallization or a known thermal crystallization method to forma crystalline semiconductor film. The crystalline semiconductor filmmakes island-like semiconductor layers 303 to 306. The island-likesemiconductor layers 303 to 306 each have a thickness of 25 to 80 nm(preferably 30 to 60 nm). No limitation is put on the choice of materialof the crystalline semiconductor film but it is preferable to usesilicon or a silicon germanium (SiGe) alloy.

When the crystalline semiconductor film is formed by lasercrystallization, a pulse oscillation type or continuous wave excimerlaser, YAG laser, or YVO₄ laser is used. Laser light emitted from alaser as those given in the above is desirably collected into a linearbeam by an optical system before irradiating the semiconductor film.Conditions of crystallization are set suitably by an operator. However,if an excimer laser is used, the pulse oscillation frequency is set to300 Hz and the laser energy density is set to 100 to 400 mJ/cm²(typically 200 to 300 mJ/cm²). If a YAG laser is used, second harmonicthereof is employed and the pulse oscillation frequency is set to 30 to300 kHz while setting the laser energy density to 300 to 600 mJ/cm²(typically 350 to 500 mJ/cm²). The laser light is collected into alinear beam having a width of 100 to 1000 μm, for example, 400 μm, toirradiate the entire substrate. The substrate is irradiated with thelinear laser light with the beams overlapping each other at an overlapratio of 50 to 90%.

Next, a gate insulating film 307 is formed so as to cover theisland-like semiconductor layers 303 to 306. The gate insulating film307 is formed from an insulating film containing silicon by plasma CVDor sputtering to a thickness of 40 to 150 nm. In this embodiment, asilicon oxynitride film having a thickness of 120 nm is used. Needlessto say, the gate insulating film is not limited to a silicon oxynitridefilm but may be a single layer or a laminate of other insulating filmscontaining silicon. For example, if a silicon oxide film is used for thegate insulating film, the film is formed by plasma CVD in which TEOS(tetraethyl orthosilicate) is mixed with O₂ and the reaction pressure isset to 40 Pa, the substrate temperature to 300 to 400° C., the frequencyis set high to 13.56 MHz, and the power density is set to 0.5 to 0.8W/cm² for electric discharge. The silicon oxide film thus formed canprovide the gate insulating film with excellent characteristics when itis subjected to subsequent thermal annealing at 400 to 500° C.

On the gate insulating film 307, a first conductive film 308 and asecond conductive film 309 for forming gate electrodes are formed. Inthis example, the first conductive film 308 is a Ta film with athickness of 50 to 100 nm and the second conductive film 309 is a W filmwith a thickness of 100 to 300 nm.

The Ta film is formed by sputtering in which Ta as a target is sputteredwith Ar. In this case, an appropriate amount of Xe or Kr is added to Arto ease the internal stress of the Ta film and thus prevent the Ta filmfrom peeling off. The resistivity of a Ta film in α phase is about 20μΩcm and is usable for a gate electrode. On the other hand, theresistivity of a Ta film in β phase is about 180 μΩcm and is notsuitable for a gate electrode. A Ta film in α phase can readily beobtained when a base with a thickness of about 10 to 50 nm is formedfrom tantalum nitride that has a crystal structure approximate to thatof the α phase Ta film.

The W film is formed by sputtering with W as a target. Alternatively,the W film may be formed by thermal CVD using tungsten hexafluoride(WF₆). In either case, the W film has to have a low resistivity in orderto use the W film as a gate electrode. A desirable resistivity of the Wfilm is 20 μΩcm or lower. The resistivity of the W film can be reducedby increasing the crystal grain size but, if there are too many impurityelements such as oxygen in the W film, crystallization is inhibited toraise the resistivity. Accordingly, when the W film is formed bysputtering, a W target with a purity of 99.9999% or 99.99% is used and agreat care is taken not to allow impurities in the air to mix in the Wfilm being formed. As a result, the W film can have a resistivity of 9to 20 μΩcm.

Although the first conductive film 308 is a Ta film and the secondconductive film 309 is a W film in this example, there is no particularlimitation. The conductive films may be formed of any element selectedfrom the group consisting of Ta, W, Ti, Mo, Al, and Cu, or of an alloymaterial or compound material mainly containing the elements listedabove. A semiconductor film, typically a polycrystalline silicon filmdoped with an impurity element such as phosphorus, may be used instead.Other desirable combinations of materials for the first and secondconductive films than the one shown in this embodiment includes:tantalum nitride (TaN) for the first conductive film 308 and W for thesecond conductive film 309; tantalum nitride (TaN) for the firstconductive film 308 and Al for the second conductive film 309; andtantalum nitride (TaN) for the first conductive film 308 and Cu for thesecond conductive film 309. (FIG. 14A)

Next, a resist mask 310 is formed to carry out first etching treatmentfor forming electrodes and wiring lines. In this embodiment, ICP(inductively coupled plasma) etching is employed in which CF₄ and Cl₂are mixed as etching gas and an RF (13.56 MHz) power of 500 W is givento a coiled electrode at a pressure of 1 Pa to generate plasma. Thesubstrate side (sample stage) also receives an RF (13.56 MHz) power of100 W so that a substantially negative self-bias voltage is applied.When the mixture of CF₄ and Cl₂ is used, the W film and the Ta film areetched to the same degree.

Under the above etching conditions, if the resist mask is properlyshape, the first conductive film and the second conductive film aretapered around the edges by the effect of the bias voltage applied tothe substrate side. The angle of the tapered portions is 15 to 45°. Inorder to etch the conductive films without leaving any residue on thegate insulating film, the etching time is prolonged by about 10 to 20%.The selective ratio of the W film to the silicon oxynitride film is 2 to4 (typically 3), and therefore a region where the silicon oxynitridefilm is exposed is etched by about 20 to 50 nm by the over-etchingtreatment. In this way, first shape conductive layers 311 to 315 (firstconductive layers 311 a to 315 a and second conductive layers 311 b to315 b) are formed from the first conductive film and the secondconductive film through the first etching treatment. At this point,regions of the gate insulating film 307 that are not covered with thefirst shape conductive layers 311 to 315 are etched and thinned by about20 to 50 nm. Further, the mask 310 is also etched by the above etchingtreatment.

First doping treatment is conducted next for doping of an impurityelement that gives the n-type conductivity. Ion doping or ion implantingis employed. In ion doping the dose is set to 1×10¹³ to 5×10¹⁴ atoms/cm²and the acceleration voltage is set to 60 to 100 keV. The impurityelement that gives the n-type conductivity is an element belonging toGroup 15, typically, phosphorus (P) or arsenic (As). Here, phosphorus(P) is used. In this case, the conductive layers 311 to 314 serve asmasks against the impurity element that gives the n-type conductivity,and first impurity regions 317 to 320 are formed in a self-aligningmariner. The first impurity regions 317 to 320 each contain the impurityelement that gives the n-type conductivity in a concentration of 1×10²⁰to 1×10²¹ atoms/cm³. (FIG. 14B)

Next, second etching treatment is conducted while leaving the resistmask 310 in place as shown in FIG. 14C. CF₄, Cl₂, and O₂ are used asetching gas to etch the W film selectively. Through the second etchingtreatment, second shape conductive layers 325 to 329 (first conductivelayers 325 a to 329 a and second conductive layers 325 b to 329 b) areformed. At this point, regions of the gate insulating film 307 that arenot covered with the second shape conductive layers 325 to 329 arefurther etched and thinned by about 20 to 50 nm.

The reaction of the W film and the Ta film to etching by the mixture gasof CF₄ and Cl₂ can be deduced from the vapor pressure of radical or ionspecies generated and of reaction products. Comparing the vapor pressureamong fluorides and chlorides of W and Ta, WF₆ that is a fluoride of Whas an extremely high vapor pressure while the others, namely, WCl₅,TaF₅, and TaCl₅ have a vapor pressure of about the same degree.Accordingly, the W film and the Ta film are both etched with the mixturegas of CF₄ and Cl₂. However, when an appropriate amount of O₂ is addedto this mixture gas, CF₄ and O₂ react to each other to be changed intoCO and F, generating a large amount of F radicals or F ions. As aresult, the W film whose fluoride has a high vapor pressure is etched atan increased etching rate. On the other hand, the etching rate of the Tafilm is not increased much when F ions are increased in number. Since Tais more easily oxidized than W, the addition of O₂ results inoxidization of the surface of the Ta film. The oxide of Ta does notreact with fluorine or chlorine and therefore the etching rate of the Tafilm is reduced further. Thus, a difference in etching rate isintroduced between the W film and the Ta film, so that the etching rateof the W film is set faster than the etching rate of the Ta film.

Then second doping treatment is conducted as shown in FIG. 15A. In thesecond doping treatment, the film is doped with an impurity element thatgives the n-type conductivity in a dose smaller than in the first dopingtreatment and at a high acceleration voltage. For example, theacceleration voltage is set to 70 to 120 keV and the dose is set to1×10¹³ atoms/cm² to form new impurity regions inside the first impurityregions that are formed in the island-like semiconductor layers in FIG.14B. While the second shape conductive layers 325 to 328 are used asmasks against the impurity element, regions under the first conductivelayers 325 a to 328 a are also doped with the impurity element. Thusformed are third impurity regions 332 to 335. The third impurity regions332 to 335 contain phosphorus (P) with a gentle concentration gradientthat conforms with the thickness gradient in the tapered portions of thefirst conductive layers 325 a to 328 a. In the semiconductor layers thatoverlap the tapered portions of the first conductive layers 325 a to 328a, the impurity concentration is slightly lower around the center thanat the edges of the tapered portions of the first conductive layers 325a to 328 a. However, the difference is very slight and almost the sameimpurity concentration is kept throughout the semiconductor layers.

Third etching treatment is then carried out as shown in FIG. 15B. CHF₆is used as etching gas, and reactive ion etching (RIE) is employed.Through the third etching treatment, the tapered portions of the firstconductive layers 325 a to 329 a are partially etched to reduce theregions where the first conductive layers overlap the semiconductorlayers. Thus formed are third shape conductive layers 336 to 340 (firstconductive layers 336 a to 340 a and second conductive layers 336 b to340 b). At this point, regions of the gate insulating film 307 that arenot covered with the third shape conductive layers 336 to 340 arefurther etched and thinned by about 20 to 50 nm.

Third impurity regions 332 to 335 are formed through the third etchingtreatment. The third impurity regions 332 to 335 consist of thirdimpurity regions 332 a to 335 a that overlap the first conductive layers336 a to 339 a, respectively, and second impurity regions 332 b to 335 beach formed between a first impurity region and a third impurity region.

As shown in FIG. 15C, fourth impurity regions 343 to 348 having theopposite conductivity type to the first conductivity type are formed inthe island-like semiconductor layers 303 and 306 for forming p-channelTFTs. The third shape conductive layers 336 b and 339 b are used asmasks against the impurity element and impurity regions are formed in aself-aligning manner. At this point, the island-like semiconductorlayers 304 and 305 for forming n-channel TFTs and third shape conductivelayer 340 are entirely covered with a resist mask 350. The impurityregions 343 to 348 have already been doped with phosphorus in differentconcentrations. The impurity regions 343 to 348 are doped with diborane(B₂H₆) through ion doping such that diborane dominates phosphorus ineach region and each region contain the impurity element in aconcentration of 2×10²⁰ to 2×10²¹ atoms/cm³.

Through the steps above, the impurity regions are formed in therespective island-like semiconductor layers. The third shape conductivelayers 336 to 339 overlapping the island-like semiconductor layersfunction as gate electrodes. The third conductive layers 340 function asgate wiring lines.

After the resist mask 350 is removed, the impurity elements used to dopethe island-like semiconductor layers in order to control theconductivity types are activated. The activation step is carried out bythermal annealing using an annealing furnace. Other activation methodsadoptable include laser annealing and rapid thermal annealing (RTA). Thethermal annealing is conducted in a nitrogen atmosphere with an oxygenconcentration of 1 ppm or less, preferably 0.1 ppm or less, at 400 to700° C., typically 500 to 600° C. In this example, the substrate issubjected to heat treatment at 500° C. for four hours. However, if thewiring line material used for the third shape conductive layers 336 to340 are weak against heat, the activation is desirably made after aninterlayer insulating film (mainly containing silicon) is formed inorder to protect the wiring lines and others. The third shape conductivelayer 340 is a gate wiring line, and a part of which functions as gateelectrode of the transistor Tr1 (not illustrated). In addition, the gatewiring line is connected to source region or drain region of thetransistor Tr3 (not illustrated).

Another heat treatment is conducted in an atmosphere containing 3 to100% hydrogen at 300 to 450° C. for one to twelve hours, therebyhydrogenating the island-like semiconductor layers. The hydrogenationsteps are to terminate dangling bonds in the semiconductor layers usingthermally excited hydrogen. Alternatively, plasma hydrogenation (usinghydrogen that is excited by plasma) may be employed.

As shown in FIG. 16A, a first interlayer insulating film 355 is formednext from a silicon oxynitride film with a thickness of 100 to 200 nm. Asecond interlayer insulating film 356 is formed thereon from an organicinsulating material. Thereafter, contact holes are formed through thefirst interlayer insulating film 355, the second interlayer insulatingfilm 356, and the gate insulating film 307. Connection wiring lines 357to 363 are formed by patterning. Reference symbol 363 denotes a powersource line and a symbol 360 denotes a signal line.

The second interlayer insulating film 356 is a film made of an organicresin. Examples of the usable organic resin include polyimide,polyamide, acrylic resin, and BCB (benzocyclobutene). Sinceplanarization is a significant aspect of the role of the secondinterlayer insulating film 356, acrylic resin that can level the surfacewell is particularly preferable. In this embodiment, the acrylic film isthick enough to eliminate the level differences caused by the TFTs. Anappropriate thickness of the film is 1 to 5 μm (preferably 2 to 4 μm).

The contact holes are formed by dry etching or wet etching, and includecontact holes reaching the impurity regions 317 to 319 having the n-typeconductivity or the impurity regions 345 and 348 having the p-typeconductivity, contact holes reaching the gate wiring lines 340, contactholes (not shown) reaching the power supply lines, and contact holes(not shown) reaching the gate electrodes.

The connection wiring lines 357 to 363 are obtained by patterning alaminate with a three-layer structure into a desired shape. The laminateconsists of a Ti film with a thickness of 100 nm, a Ti-containingaluminum film with a thickness of 300 nm, and a Ti film with a thicknessof 150 nm which are successively formed by sputtering. Other conductivefilms may of course be used.

The pixel electrode 365 in contact with the connecting wiring (drainwiring) 362 is formed by patterning. The connecting wiring comprises thesource wiring and the drain wiring. The source wiring designates awiring connected to the source region of the active layer, and the drainwiring designates a wiring connected to the drain region.

The pixel electrode 365 in this example is obtained by patterning an ITOfilm with a thickness of 110 nm. A contact is made by arranging thepixel electrode 365 so as to touch the connection wiring line 362. Thepixel electrode may instead be formed of a transparent conductive filmin which indium oxide is mixed with 2 to 20% zinc oxide (ZnO). The pixelelectrode 365 serves as an anode of an OLED. (FIG. 16A)

Next, as shown in FIG. 16B, an insulating film containing silicon (asilicon oxide film, in this embodiment) is formed to a thickness of 500nm and an aperture is opened in the film at a position corresponding tothe position of the pixel electrode 365. A third interlayer insulatingfilm 366 functioning as a bank is thus formed. The aperture is formedusing wet etching, thereby readily forming tapered side walls. If theside wall of the aperture is not smooth enough, the level difference canmake degradation of an organic light-emitting layer into a seriousproblem. Therefore attention must be paid.

An organic light-emitting layer 367 and a cathode (MgAg electrode) 368are formed by vacuum evaporation successively without exposing thesubstrate to the air. The thickness of the organic light-emitting layer367 is set to 80 to 200 nm (typically 100 to 120 nm). The thickness ofthe cathode 368 is set to 180 to 300 nm (typically 200 to 250 nm).

In this step, the organic light-emitting layer and the cathode areformed in a pixel for red light, then in a pixel for green light, andthen in a pixel for blue light. The organic light-emitting layers havelow resistivity to solutions, inhibiting the use of photholithography.Therefore, an organic light-emitting layer of one color cannot be formedtogether with an organic light-emitting layer of another color. Thenorganic light-emitting layers and cathodes are selectively formed inpixels of one color while covering pixels of the other two colors with ametal mask.

To elaborate, first, a mask that covers all the pixels except pixels forred light is set and the organic light-emitting layers for emitting redlight are selectively formed using the mask. Then a mask that covers allthe pixels except pixels for green light is set and the organiclight-emitting layers for emitting green light are selectively formedusing the mask. Lastly, a mask that covers all the pixels except pixelsfor blue light is set and the organic light-emitting layers for emittingblue light are selectively formed using the mask. Although differentmasks are used in the description here, the same mask may be used threetimes for forming the organic light-emitting layers of three colors.

Formed here are three types of OLED in accordance with R, G, and B.Instead, a white light-emitting OLED combined with color filters, a bluelight or bluish green light-emitting element combined with fluorophors(fluorescent color conversion layers: CCM), or overlapped RGB OLED witha cathode (opposite electrode) formed of a transparent electrode may beused.

A known material can be used for the organic light-emitting layer 367. Apreferable known material is an organic material, considering thedriving voltage. For example, the organic light-emitting layer has afour-layer structure consisting of a hole injection layer, a holetransporting layer, a light-emitting layer, and an electron injectionlayer.

The cathode 368 is formed next. This example uses MgAg for the cathode368 but it is not limited thereto. Other known materials may be used forthe cathode 368.

The overlapping portion, which is comprised the pixel electrode 365, theorganic light-emitting layer 367 and the cathode 368, corresponds toOLED 375.

Next, the protective electrode 369 is formed by an evaporation method.The protective electrode 369 may be formed in succession forming thecathode 368 without exposing the device to the air. The protectiveelectrode 369 has an effect on protect the organic light-emitting layer367 from moisture and oxygen.

The protective electrode 369 also prevents degradation of the cathode368. A typical material of the protective electrode is a metal filmmainly containing aluminum. Other material may of course be used. Sincethe organic light-emitting layer 367 and the cathode 368 are extremelyweak against moisture, the organic light-emitting layer 367, the cathode368, and the protective electrode 369 are desirably formed in successionwithout exposing them to the air. It is preferable to protect theorganic light-emitting layer from the outside air.

Lastly, a passivation film 370 is formed from a silicon nitride filmwith a thickness of 300 nm. The passivation film 370 protects theorganic compound layer 367 from moisture and the like, thereby furtherenhancing the reliability of the OLED. However, the passivation film 370may not necessarily be formed.

A light-emitting device structured as shown in FIG. 16B is thuscompleted. Reference symbol 371 denotes p-channel TFT of the drivingcircuit, 372, n-channel TFT of driving circuit, 373, the transistor Tr4,and 374, the transistor Tr2.

The light-emitting device of this example exhibits very high reliabilityand improved operation characteristics owing to placing optimallystructured TFTs in not only the pixel portion but also in the drivingcircuits. In the crystallization step, the film may be doped with ametal catalyst such as Ni to enhance the crystallinity. By enhancing thecrystallinity, the drive frequency of the signal line driving circuitcan be set to 10 MHz or higher.

In practice, the device reaching the state of FIG. 16B is packaged(enclosed) using a protective film that is highly airtight and allowslittle gas to transmit (such as a laminate film and a UV-curable resinfilm) or a light-transmissive seal, so as to further avoid exposure tothe outside air. A space inside the seal may be set to an inertatmosphere or a hygroscopic substance (barium oxide, for example) may beplaced there to improve the reliability of the OLED.

After securing the airtightness through packaging or other processing, aconnector is attached for connecting an external signal terminal with aterminal led out from the elements or circuits formed on the substrate.

By following the process shown in this embodiment, the number of photomasks needed in manufacturing a light-emitting device can be reduced. Asa result, the process is cut short to reduce the manufacture cost andimprove the yield.

The structure of this example can be combined freely with Embodiment 1through 6.

Embodiment 8

This embodiment explains, in a top view, a pixel as formed in Embodiment2. FIG. 17 shows a top view of a pixel of this embodiment. Note thatFIG. 17 corresponds to a top view of a pixel of upon completing theprocess of FIG. 16A. In FIG. 17, various insulation films, such asinterlayer insulation films and gate insulation films, are omitted inorder to clarify the position of wirings and semiconductor layers. Thewiring formed in the same layer is shown with the same hatching.

The sectional view taken on a broken line A-A′ in FIG. 17 corresponds toa part A-A′in FIG. 16A. FIG. 18 shows a sectional view taken on a brokenline B-B′ in FIG. 17.

The pixel shown in FIG. 17 has a wiring 360 (Si) serving as a signalline, a first scanning line 380 (Gaj), a second scanning line 381 (Gbj)and a power line 363 (Vj), each one in the number. The first scanningline 380 has portions 382, 327 respectively corresponding to the gateelectrodes of the transistors Tr3 and Tr4.

One of the source and drain regions of the transistor Tr3 is connectedto the signal line 360 while the other is connected to a gate wiring 340through a wiring 383. The gate wiring 340 has a portion 384 serving as agate electrode of the transistor Tr1.

Meanwhile, one of the source and drain regions of the transistor Tr4 isconnected to the signal line 360 while the other is connected to thedrain region of the transistor Tr1 and to the source region of thetransistor Tr2 through a wiring 361.

The source region of the transistor Tr1 is connected to the power line363. The drain region of the transistor Tr2 is connected to a pixelelectrode 365 through a wiring 362.

The second scanning line 381 has a portion 328 serving as a gateelectrode of the is transistor Tr2.

The power line 363 overlaps with the gate wiring 340 through asandwiched first and second interlayer film. The gate wiring 340overlaps with a capacitance wiring 385 formed by a semiconductor filmadded with an impurity, through a sandwiched gate insulation film (notshown). The power line 363 and the capacitance wiring 385 are connectedtogether through a contact hole. Note that the region, where the gatewiring 340 overlaps with the capacitance wiring 385 through a sandwichedgate insulation film, corresponds to a storage capacitor 386.Furthermore, the region, where the power line 363 overlaps with the gatewiring 340 through the sandwiched first and second interlayer film, maybe used as a storage capacitor.

By forming the power line 363 at beneath a partition wall (bank)demarcating between the pixels, the storage capacitor and power line canbe formed without reducing the opening ratio.

The pixel top view shown in this embodiment is a mere one structuralexample of the present invention. The pixel shown in the top view ofFIG. 17 is not limited to the structure shown in this embodiment. Notethat this example can be carried out in free combination with Embodiment1-7.

Embodiment 9

This embodiment explains, in a top view, the pixel shown in FIG. 8. FIG.19 shows a top view of a pixel in the present embodiment. Note that FIG.17 corresponds to a top view of a pixel in a stage of after forming apixel electrode but before depositing an organic light-emitting layer.In FIG. 19, various insulation films including interlayer and gateinsulation films are omitted in order to clarify the position of awiring and semiconductor layer. The wiring formed in the same layer isshown with hatching.

The pixel shown in FIG. 19 has a wiring 560 (Si) serving as a signalline, a scanning line 580 (Gj) and a power line 563 (Vi), one each inthe number. The scanning line 580 has portions 582, 527, 528respectively corresponding to the gate electrodes of the transistorsTr3, Tr4 and Tr2.

One of the source and drain regions of the transistor Tr3 is connectedto the signal line 560 while the other is connected to a gate wiring 540through a wiring 583. The gate wiring 540 has a portion 584 serving as agate electrode of the transistor Tr1.

Meanwhile, one of the source and drain regions of the transistor Tr4 isconnected to the signal line 560 while the other is connected to thedrain region of the transistor Tr1 and to the source region of thetransistor Tr2 through a wiring 561.

The source region of the transistor Tr1 is connected to the power line563. The drain region of the transistor Tr2 is connected to a pixelelectrode 565 through a wiring 562.

The power line 563 overlaps with the gate wiring 540 through asandwiched first and second interlayer film. The gate wiring 540overlaps with a capacitance wiring 585 formed by a semiconductor filmadded with an impurity, through a sandwiched gate insulation film (notshown). The power line 563 and the capacitance wiring 585 are connectedtogether through a contact hole. Note that the region, where the gatewiring 540 overlaps with the capacitance wiring 585 through a sandwichedgate insulation film, corresponds to a storage capacitor 586.Furthermore, the region, where the power line 563 overlaps with the gatewiring 540 through the sandwiched first and second interlayer film, maybe used as a storage capacitor.

By forming the power line 563 at beneath a partition wall (bank)demarcating between the pixels, the storage capacitor and power line canbe formed without reducing the opening ratio.

The pixel shown in the top view of this embodiment is a mere onestructural example of the present invention. The pixel shown in the topview of FIG. 19 is not limited to the structure shown in the presentembodiment. Note that this embodiment can be carried out in freecombination with Embodiment 1-7.

Embodiment 10

This embodiment describes structures of driving circuits (a signal linedriving circuit and a first scanning line driving circuit) in alight-emitting device of the present invention which is driven by usinga digital video signal.

FIG. 20 is a block diagram showing the structure of a signal linedriving circuit 601. Reference symbol 602 denotes a shift register, 603,a memory circuit A, 604, a memory circuit B, and 605, a constant currentcircuit.

Clock signals CLK and start pulse signals SP are inputted to the shiftregister 602. Digital video signals are inputted to the memory circuit A603 and latch signals are inputted to the memory circuit B 604. Theconstant current circuit 605 outputs a constant signal current Ic, whichis inputted to signal lines.

FIG. 21 shows a more detailed structure of the signal line drivingcircuit 601.

The shift register 602 generates timing signals in response to clocksignals CLK and start pulse signals SP inputted from given wiring lines.The timing signals are respectively inputted to a plurality of latches A(LATA_1 to LATA_x) of the memory circuit A 603. The timing signalsgenerated in the shift register 602 may be buffered and amplified by abuffer or the like before inputting the signals to the plural latches A(LATA_1 to LATA_x) of the memory circuit A 603.

When timing signals are inputted to the memory circuit A 603, in syncwith the timing signals, digital video signals equivalent to one bitwhich are inputted to a video signal line 610 are sequentially writtenin the plural latches A (LATA_1 to LATA_x) to be stored therein.

In this embodiment, digital video signals are sequentially inputted tothe plural latches A (LATA_1 to LATA_x) of the memory circuit A 603 wheninputting digital video signals into the memory circuit A 603. However,the present invention is not limited thereto. The present invention mayemploy a so-called division driving in which the plural stages of lathesof the memory circuit A 603 are divided into a few groups and digitalvideo signals are inputted to the respective groups simultaneously. Thenumber of groups in division driving is referred to as number ofdivision. For example, if four stages of latches make one group, then itis four divisions driving.

The time required for completing writing digital video signals once intoall stages of latches of the memory circuit A 603 is called a lineperiod. In practice, sometimes the line period defined as above plus ahorizontal retrace period is regarded as a line period.

Upon completion of one line period, latch signals are supplied to aplurality of latches B (LATB_1 to LATB_x) of the memory circuit B 604through a latch signal line 609. At this instant, the digital videosignals that have been held in the plural latches A (LATA_1 to LATA_x)of the memory circuit A 603 are sent to the plural latches B (LATB_1 toLATB_x) of the memory circuit B 604 all at once to be written and heldtherein.

Having sent the digital video signals to the memory circuit B 604, thememory circuit A 603 now receives the next supply of digital videosignals equivalent to one bit so that the digital video signals aresequentially written in response to timing signals from the shiftregister 602.

After one line period is thus started for the second time, the digitalvideo signals written and held in the memory circuit B 604 are inputtedto the constant current circuit 605.

The constant current circuit 605 has a plurality of current settingcircuits (C1 to Cx). When digital video signals are respectivelyinputted to the current setting circuits (C1 to Cx), information of‘0’or ‘1’ contained in the digital video signals determines whether aconstant current Ic flows in the signal line or the signal line receivesthe electric potential of power supply lines V1 to Vx.

FIG. 22 shows an example of the specific structure for the currentsetting circuit C1. This structure is shared by the current settingcircuits C2 to Cx.

The current setting circuit C1 has a constant current source 631, fourtransmission gates SW1 to SW4, and two inverters Inb1 and Inb2. Atransistor 650 of the constant current source 631 has the same polarityas those of transistors Tr1 and Tr2 of each pixel.

Digital video signals outputted from the LATB_1 of the memory circuit B604 are used to control switching of SW1 to SW4. Digital video signalsinputted to SW1, SW3 are inverted by Inb1 and Inb2, and the inverteddigital video signals are inputted to SW2 and SW4. Accordingly, SW2 andSW4 are OFF when SW1 and SW3 are ON and, when SW1 and SW3 are OFF, SW2and SW4 are ON.

When SW1 and SW3 are ON, the current Ic having a given value other than0 is inputted from the constant current source 631 to a signal line S1through SW1 and SW3. On the other hand, when SW2 and SW4 are ON, thecurrent Ic from the constant current source 631 is dropped to the groundthrough SW2 and the power supply electric potential of the power supplylines Vi to Vx is given to the signal line S1 to set Ic nearly equal to0 through SW4.

Back to FIG. 21, the operation described above is simultaneouslyconducted in all of the current setting circuits (C1 to Cx) of theconstant current circuit 605 in one line period. Therefore, the value ofthe signal current Ic to be inputted is determined for the respectivesignal lines by digital video signals.

The structure of the first scanning line driving circuit is describednext.

FIG. 23 is a block diagram showing the structure of a first scanningline driving circuit 641.

The first scanning line driving circuit 641 has a shift register 642 anda buffer 643. In some cases, the first scanning line driving circuit mayhave a level shifter.

In the first scanning line driving circuit 641, timing signals aregenerated upon input of clock signals CLK and start pulse signals SP tothe shift register 642. The timing signals generated are buffered andamplified by the buffer 643 and then the signals are supplied toassociate scanning lines.

One scanning line is connected to gate electrodes of first switchingtransistors and second switching transistors of one line of pixels.Since the first switching transistors and second switching transistorsof one line of pixels have to be turned ON all at once, the buffer 643used is capable of causing a large amount of current to flow.

Structures of the driving circuits used in the present invention are notlimited to those shown in this embodiment. The structure of the constantcurrent circuit of this example is not limited to the one illustrated inFIG. 22. The constant current circuit used in the present invention canhave any structure as long as it can cause the signal current Ic whosevalue is chosen from two values by a digital video signal to flow into asignal line.

Further, the second scanning driving circuit may have a same structureas that of the first scanning driving circuit.

The structure of this embodiment can be combined freely with Embodiments1 through 9.

Embodiment 11

This embodiment explains a configuration of a signal line drive circuitpossessed by the light-emitting device of the present invention to bedriven with analog video signals. Note that the configuration of ascanning line drive circuit can use the configuration shown in FIG. 23and hence is herein omitted.

FIG. 24A shows a block diagram of a signal line drive circuit 401 ofthis embodiment 402 shows a shift register, 403 a buffer, 404 a samplingcircuit and 405 a current converting circuit.

The shift register 402 is inputted with a clock signal (CLK) and a startpulse signal (SP). When a clock signal (CLK) and start pulse signal (SP)is inputted to the shift register 402, a timing signal is generated.

The generated timing signal is amplified or buffer-amplified in thebuffer 403 and then inputted to the sampling circuit 404. Incidentally,a level shifter may be provided in place of the buffer, to amplify thetiming signal. Otherwise, both a buffer and a level shifter may beprovided.

FIG. 24B shows a concrete configuration of the sampling circuit 404 andcurrent converting circuit 405. The sampling circuit 404 at its terminal410 is connected to a buffer 403.

The sampling circuit 404 is provided with a plurality of switches 411.An analog video signal is inputted to the sampling circuit 404 through avideo signal line 406. The switches 411 sample the analog video signalsynchronously with the timing signal and inputs it to the rear-stagedcurrent converting circuit 405. Incidentally, although FIG. 24B showsonly the current converting circuit connected to one of the switches 411possessed by the sampling circuit 404, it is assumed that currentconverting circuits 405 as shown in FIG. 24B are respectively connectedto the rear stages of the switches 411.

Incidentally, although the example uses only one transistor on theswitch 411, the switch 411 is satisfactorily sampling an analog videosignal synchronously with the timing signal and not limited to theconfiguration of this embodiment.

The sampled analog video signal is inputted to a current output circuit412 possessed by the current converting circuit 405. A current outputcircuit 412 outputs a current (signal current) in a value commensuratewith a voltage of the input video signal. Incidentally, although, inFIG. 24, the current output circuit is formed with using an amplifierand transistor, the present invention is not limited to theconfiguration but may be a circuit to output a current in a valuecommensurate with an input signal voltage.

The signal current is inputted to a reset circuit 417 also possessed bythe current converting circuit 405. The reset circuit 407 has two analogswitches 413, 414, an inverter 416 and a power source 415.

An analog switch 414 is inputted with a reset signal (Res) while ananalog switch 413 is inputted with a reset signal (Res) inverted by aninverter 416. The analog switch 413 and the analog switch 414 operaterespectively synchronously with the inverted reset signal and the resetsignal, so that, when one is on, the other is off.

When the analog switch 413 is on, the signal current is inputted to thecorresponding signal line. Conversely, when the analog switch 414 is on,the potential of a power source 415 is provided to the signal line,thereby resetting the signal line. Incidentally, the potential on thepower source 415 desirably has nearly the same height as the potentialon a power line provided on the pixel. The current flowing through thesignal line while the signal line is being reset is preferably as closeas possible to 0.

The signal line is desirably reset in a fly-back period. However, resetis possible as required in a period other than a fly-back period if in aperiod other than the period of displaying an image.

Incidentally, the signal line drive circuit and first scanning linedrive circuit for driving the light-emitting device of the presentinvention is not limited to the configuration shown in this example. Theconfiguration of this example can be carried out in free combinationwith Embodiments 1-10.

Embodiment 12

In this embodiment, an external light-emitting quantum efficiency can beremarkably improved by using an organic light-emitting material by whichphosphorescence from a triplet exciton can be employed for emitting alight. As a result, the power consumption of the OLED can be reduced,the lifetime of the OLED can be elongated and the weight of the OLED canbe lightened.

The following is a report where the external light-emitting quantumefficiency is improved by using the triplet exciton (T. Tsutsui, C.Adachi, S. Saito, Photochemical processes in Organized MolecularSystems, ed. K. Honda, (Elsevier Sci. Pub., Tokyo, 1991) p. 437).

The molecular formula of an organic light-emitting material (coumarinpigment) reported by the above article is represented as follows.

-   (M. A. Baldo, D. F. O'Brien, Y. You, A. Shoustikov, S. Sibley, M. E.    Thompson, S. R. Forrest, Nature 395 (1998) p. 151)

The molecular formula of an organic light-emitting material (Pt complex)reported by the above article is represented as follows.

-   (M. A. Baldo, S. Lamansky, P. E. Burrows, M. E. Thompson, S. R.    Forrest, Appl. Phys. Lett., 75 (1999) p. 4.)-   (T. Tsutsui, M.-J. Yang, M. Yahiro, K. Nakamura, T. Watanabe, T.    Tsuji, Y. Fukuda, T. Wakimoto, S. Mayaguchi, Jpn, Appl. Phys., 38    (12B) (1999) L1502)

The molecular formula of an organic light-emitting material (Ir complex)reported by the above article is represented as follows.

As described above, if phosphorescence from a triplet exciton can be putto practical use, it can realize the external light-emitting quantumefficiency three to four times as high as that in the case of usingfluorescence from a singlet exciton in principle.

The structure according to this embodiment can be freely implemented incombination of any structures of Embodiment 1 through 11.

Embodiment 13

In this embodiment, an example of manufacturing the light-emittingdevice using the present invention is described with reference to FIGS.25A to 25C.

FIG. 25 is a top view of the light-emitting device which is formedaccording as the element substrate with the transistor is sealed bysealing materials, FIG. 25 B is a cross sectional view taken along witha line A-A′ of FIG. 25A, and FIG. 25C is a cross sectional view takenalong with a line B-B′ of FIG. 25A.

A seal member 4009 is provided so as to surround a pixel portion 4002, asignal line driver circuit 4003, and the first, second scanning linedriver circuits 4004 a, 4004 b, which are provided on a substrate 4001.Further, a sealing material 4008 is provided on the pixel portion 4002,the signal line driver circuit 4003, and the first, the second scanningline driver circuits 4004 a, 4004 b. Thus, the pixel portion 4002, thesignal line driver circuit 4003, and the first, the second scanning linedriver circuits 4004 a, 4004 b are sealed by the substrate 4001, theseal member 4009 and the sealing material 4008 together with a filler4210.

Further, the pixel portion 4002, the signal line driver circuit 4003,and the first, the second scanning line driver circuits 4004 a, 4004 b,which are provided on the substrate 4001, have a plurality of TFTs. InFIG. 25B, a driver circuit TFT (Here, an n-channel TFT and a p-channelTFT are shown in the figure.) 4201 included in the signal line drivercircuit 4003 and a transistor Tr2 4202 included in the pixel portion4002, which are formed on a base film 4010, are typically shown.

In this embodiment, the p-channel TFT or the n-channel TFT manufacturedby a known method is used as the driving TFT 4201, and the p-channel TFTmanufactured by a known method is used as the transistor Tr2 4202.Further, the pixel portion 4002 is provided with a storage capacitor(not illustrated).

An interlayer insulating film (leveling film) 4301 is formed on thedriving TFT 4201 and the transistor Tr2 4202, and a pixel electrode(anode) 4203 electrically connected to a drain of the transistor Tr24202 is formed thereon. A transparent conductive film having a largework function is used for the pixel electrode 4203. A compound of indiumoxide and tin oxide, a compound of indium oxide and zinc oxide, zincoxide, tin oxide or indium oxide can be used for the transparentconductive film. The above transparent conductive film added withgallium may also be used.

Then, an insulating film 4302 is formed on the pixel electrode 4203, andthe insulating film 4302 is formed with an opening portion on the pixelelectrode 4203. In this opening portion, an organic light-emitting layer4204 is formed on the pixel electrode 4203. A known organiclight-emitting material or inorganic light-emitting material may be usedfor the organic light-emitting layer 4204. Further, there exist a lowmolecular weight (monomer) material and a high molecular weight(polymer) material as the organic light-emitting materials, and both thematerials may be used.

A known evaporation technique or application technique may be used as amethod of forming the organic light-emitting layer 4204. Further, thestructure of the organic light-emitting layer may take a laminationstructure or a single layer structure by freely combining a holeinjecting layer, a hole transporting layer, a light-emitting layer, anelectron transporting layer and an electron injecting layer.

A cathode 4205 made of a conductive film having light-shielding property(typically, conductive film containing aluminum, copper or silver as itsmain constituent or lamination film of the above conductive film andanother conductive film) is formed on the organic light-emitting layer4204. Further, it is desirable that moisture and oxygen that exist on aninterface of the cathode 4205 and the organic light-emitting layer 4204are removed as much as possible. Therefore, such a device is necessarythat the organic light-emitting layer 4204 is formed in a nitrogen orrare gas atmosphere, and then, the cathode 4205 is formed withoutexposure to oxygen and moisture, in this example, the above-describedfilm deposition is enabled by using a multi-chamber type (cluster tooltype) film forming device. In addition, a predetermined voltage is givento the cathode 4205.

As described above, an OLED 4303 constituted of the pixel electrode(anode) 4203, the organic light-emitting layer 4204 and the cathode 4205is formed. Further, a protective film 4209 is formed on the insulatingfilm 4302 so as to cover the OLED 4303. The protective film 4209 iseffective in preventing oxygen, moisture and the like from permeatingthe OLED 4303.

Reference symbol 4005 a denotes a wiring drawn to be connected to thepower supply line, and the wiring 4005 a is electrically connected to asource region of the transistor Tr2 4202. The drawn wiring 4005 a passesbetween the seal member 4009 and the substrate 4001, and is electricallyconnected to an FPC wiring 4206 of an FPC 4006 through an anisotropicconductive film 4300.

A glass material, a metal material (typically, stainless material), aceramics material or a plastic material (including a plastic film) canbe used for the sealing material 4008. As the plastic material, an FRP(fiberglass-reinforced plastics) plate, a PVF (polyvinyl fluoride) film,a Mylar film, a polyester film or an acrylic resin film may be used.Further, a sheet with a structure in which an aluminum foil issandwiched with the PVF film or the Mylar film can also be used.

However, in the case where the light from the OLED is emitted toward thecover member side, the cover member needs to be transparent. In thiscase, a transparent substance such as a glass plate, a plastic plate, apolyester film or an acrylic film is used.

Further, in addition to an inert gas such as nitrogen or argon, anultraviolet curable resin or a thermosetting resin may be used as thefiller 4210, so that PVC (polyvinyl chloride), acrylic, polyimide, epoxyresin, silicone resin, PVB (polyvinyl butyral) or EVA (ethylene vinylacetate) can be used. In this example, nitrogen is used for the filler.

Moreover, a concave portion 4007 is provided on the surface of thesealing material 4008 on the substrate 4001 side, and a hygroscopicsubstance or a substance that can absorb oxygen 4207 is arranged thereinin order that the filler 4210 is made to be exposed to the hygroscopicsubstance (preferably, barium oxide) or the substance that can absorboxygen. Then, the hygroscopic substance or the substance that can absorboxygen 4207 is held in the concave portion 4007 by a concave portioncover member 4208 such that the hygroscopic substance or the substancethat can absorb oxygen 4207 is not scattered. Note that the concaveportion cover member 4208 has a fine mesh form, and has a structure inwhich air and moisture are penetrated while the hygroscopic substance orthe substance that can absorb oxygen 4207 is not penetrated. Thedeterioration of the OLED 4303 can be suppressed by providing thehygroscopic substance or the substance that can absorb oxygen 4207.

As shown in FIG. 25C, the pixel electrode 4203 is formed, and at thesame time, a conductive film 4203 a is formed so as to contact the drawnwiring 4005 a.

Further, the anisotropic conductive film 4300 has conductive filler 4300a. The conductive film 4203 a on the substrate 4001 and the FPC wiring4301 on the FPC 4006 are electrically connected to each other by theconductive filler 4300 a by heat-pressing the substrate 4001 and the FPC4006.

Note that this embodiment can be implemented by being freely combinedwith Embodiment 1 through 12.

Embodiment 14

The light-emitting device using the OLED is of the self-emission type,and thus exhibits more excellent recognizability of the displayed imagein a light place as compared to the liquid crystal display device.Furthermore, the light-emitting device has a wider viewing angle.Accordingly, the light-emitting device can be applied to a displayportion in various electronic devices.

Such electronic devices using a light-emitting device of the presentinvention include a video camera, a digital camera, a goggles-typedisplay (head mount display), a navigation system, a sound reproductiondevice (a car audio equipment and an audio set), a lap-top computer, agame machine, a portable information terminal (a mobile computer, amobile telephone, a portable game machine, an electronic book, or thelike), an image reproduction apparatus including a recording medium(more specifically, an apparatus which can reproduce a recording mediumsuch as a digital versatile disc (DVD) and so forth, and includes adisplay for displaying the reproduced image), or the like. Inparticular, in the case of the portable information terminal, use of thelight-emitting device is preferable, since the portable informationterminal that is likely to be viewed from a tilted direction is oftenrequired to have a wide viewing angle. FIG. 26 respectively showsvarious specific examples of such electronic devices.

FIG. 26A illustrates an organic light emitting display device whichincludes a casino 2001, a support table 2002, a display portion 2003, aspeaker portion 2004, a video input terminal 2005 or the like. Thepresent invention is applicable to the display portion 2003. Thelight-emitting device is of the self-emission-type and thereforerequires no backlight. Thus, the display portion thereof can have athickness thinner than that of the liquid crystal display device. Theorganic light emitting display device is including the entire displaydevice for displaying information, such as a personal computer, areceiver of TV broadcasting and an advertising display.

FIG. 26B illustrated a digital still camera which includes a main body2101, a display portion 2102, an image receiving portion 2103, anoperation key 2104, an external connection port 2105, a shutter 2106, orthe like. The light-emitting device in accordance with the presentinvention can be used as the display portion 2102.

FIG. 26C illustrates a lap-top computer which includes a main body 2201,a casing 2202, a display portion 2203, a keyboard 2204, an externalconnection port 2205, a pointing mouse 2206, or the like. Thelight-emitting device in accordance with the present invention can beused as the display portion 2203.

FIG. 26D illustrated a mobile computer which includes a main body 2301,a display portion 2302, a switch 2303, an operation key 2304, aninfrared port 2305, or the like. The light-emitting device in accordancewith the present invention can be used as the display portion 2302.

FIG. 26E illustrates a portable image reproduction apparatus including arecording medium (more specifically, a DVD reproduction apparatus),which includes a main body 2401, a casing 2402, a display portion A2403, another display portion B 2404, a recording medium (DVD or thelike) reading portion 2405, an operation key 2406, a speaker portion2407 or the like. The display portion A 2403 is used mainly fordisplaying image information, while the display portion B 2404 is usedmainly for displaying character information. The light-emitting devicein accordance with the present invention can be used as these displayportions A 2403 and B 2404. The image reproduction apparatus including arecording medium further includes a game machine or the like.

FIG. 26F illustrates a goggle type display (head mounted display) whichincludes a main body 2501, a display portion 2502, arm portion 2503 orthe like. The light-emitting device in accordance with the presentinvention can be used as the display portion 2502.

FIG. 26G illustrates a video camera which includes a main body 2601, adisplay portion 2602, a casing 2603, an external connecting port 2604, aremote control receiving portion 2605, an image receiving portion 2606,a battery 2607, a sound input portion 2608, an operation key 2609, orthe like. The light-emitting device in accordance with the presentinvention can be used as the display portion 2602.

FIG. 26H illustrates a mobile telephone which includes a main body 2701,a casing 2702, a display portion 2703, a sound input portion 2704, asound output portion 2705, an operation key 2706, an external connectingport 2707, an antenna 2708, or the like. The light-emitting device inaccordance with the present invention can be used as the display portion2703. Note that the display portion 2703 can reduce power consumption ofthe mobile telephone by displaying white-colored characters on ablack-colored background.

When the brighter luminance of light emitted from the organiclight-emitting material becomes available in the future, thelight-emitting device in accordance with the present invention will beapplicable to a front-type or rear-type projector in which lightincluding output image information is enlarged by means of lenses or thelike to be projected.

The aforementioned electronic devices are more likely to be used fordisplay information distributed through a telecommunication path such asInternet, a CATV (cable television system), and in particular likely todisplay moving picture information, The light-emitting device issuitable for displaying moving pictures since the organic light-emittingmaterial can exhibit high response speed.

A portion of the light-emitting device that is emitting light consumespower, so it is desirable to display information in such a manner thatthe light-emitting portion therein becomes as small as possible.Accordingly, when the light-emitting device is applied to a displayportion which mainly displays character information, e.g., a displayportion of a portable information terminal, and more particular, aportable telephone or a sound reproduction device, it is desirable todrive the light-emitting device so that the character information isformed by a light-emitting portion while a non-emission portioncorresponds to the background.

As set forth above, the present invention can be applied variously to awide range of electronic devices in all fields. The electronic device inthis embodiment can be obtained by utilizing a light-emitting devicehaving the configuration in which the structures in Embodiment 1 through13 are freely combined.

With the above structure, the light-emitting device of the presentinvention can obtain a constant brightness without being affected bytemperature change. Meanwhile, in the case of providing an OLED havingorganic light-emitting materials different between colors in colordisplay, it is possible to prevent the impossibility to obtain a desiredcolor due to random variation in OLED brightness of the colors withtemperature.

1. A semiconductor device comprising: at least one pixel comprising: agate wiring; an insulating film; a film; a first transistor; a secondtransistor; a third transistor; a fourth transistor; a power line; asignal line; and a scanning line; wherein the film is formed of the samematerial as a semiconductor film of the first transistor, wherein thegate wiring is formed over the film with the insulating film interposedtherebetween, wherein gate electrodes of said third and fourthtransistors are electrically connected to said scanning line, whereinone of source and drain regions of said third transistor is electricallyconnected to said signal line, wherein one of source and drain regionsof said fourth transistor is electrically connected to a drain region ofsaid first transistor, wherein a source region of said first transistoris electrically connected to said power line, and wherein one of sourceand drain regions of said second transistor is electrically connected tosaid drain region of said first transistor.
 2. The semiconductor deviceaccording to claim 1, wherein said third and fourth transistors are ofreverse polarity to said second transistor.
 3. The semiconductor deviceaccording to claim 1, wherein said third and fourth transistors are ofthe same polarity.
 4. The semiconductor device according to claim 1,wherein said first and second transistors are of the same polarity. 5.The electronic device comprising the semiconductor device according toclaim 1, wherein said electronic device is selected from the groupconsisting of a light emitting display device, a digital still camera, alap-top computer, a mobile computer, a portable image reproductionapparatus, a goggle type display, a video camera, and a mobiletelephone.
 6. A semiconductor device comprising: at least one pixelcomprising: a gate wiring; an insulating film; a film; a firsttransistor; a second transistor; a third transistor; a fourthtransistor; a power line; a signal line; and a scanning line; whereinthe film is formed of the same material as a semiconductor film of thefirst transistor, wherein the gate wiring is formed over the film withthe insulating film interposed therebetween, wherein gate electrodes ofsaid third and fourth transistors are electrically connected to saidscanning line, wherein one of source and drain regions of said thirdtransistor is electrically connected to said signal line, wherein one ofsource and drain regions of said fourth transistor is electricallyconnected to a drain region of said first transistor, wherein a sourceregion of said first transistor is electrically connected to said powerline, wherein one of source and drain regions of said second transistoris electrically connected to said drain region of said first transistor,and wherein each of the first transistor, the second transistor, thethird transistor and the fourth transistor is a thin-film transistorusing polysilicon.
 7. The semiconductor device according to claim 6,wherein said third and fourth transistors are of reverse polarity tosaid second transistor.
 8. The semiconductor device according to claim6, wherein said third and fourth transistors are of the same polarity.9. The semiconductor device according to claim 6, wherein said first andsecond transistors are of the same polarity.
 10. The electronic devicecomprising the semiconductor device according to claim 6, wherein saidelectronic device is selected from the group consisting of a lightemitting display device, a digital still camera, a lap-top computer, amobile computer, a portable image reproduction apparatus, a goggle typedisplay, a video camera, and a mobile telephone.
 11. A semiconductordevice comprising: at least one pixel comprising: a gate wiring; aninsulating film; a film; a first transistor; a second transistor; athird transistor; a fourth transistor; a power line; a signal line; anda scanning line; wherein the film is formed of the same material as asemiconductor film of the first transistor, wherein the gate wiring isformed over the film with the insulating film interposed therebetween,wherein gate electrodes of said third and fourth transistors areelectrically connected to said scanning line, wherein one of source anddrain regions of said third transistor is electrically connected to saidsignal line, wherein one of source and drain regions of said fourthtransistor is electrically connected to a drain region of said firsttransistor, wherein one of source and drain regions of said secondtransistor is electrically connected to said drain region of said firsttransistor, and wherein the power line is a laminate film comprising: afirst layer comprising titanium, the first layer being electricallyconnected to a source region of said first transistor; a second layercomprising aluminum, over the first layer; and a third layer comprisingtitanium, over the second layer.
 12. The semiconductor device accordingto claim 11, wherein said third and fourth transistors are of reversepolarity to said second transistor.
 13. The semiconductor deviceaccording to claim 11, wherein said third and fourth transistors are ofthe same polarity.
 14. The semiconductor device according to claim 11,wherein said first and second transistors are of the same polarity. 15.The electronic device comprising the semiconductor device according toclaim 11, wherein said electronic device is selected from the groupconsisting of a light emitting display device, a digital still camera, alap-top computer, a mobile computer, a portable image reproductionapparatus, a goggle type display, a video camera, and a mobiletelephone.
 16. A semiconductor device comprising: at least one pixelcomprising: a gate wiring; an insulating film; a film; a firsttransistor; a second transistor; a third transistor; a fourthtransistor; a power line; a signal line; and a scanning line; whereinthe film is formed of the same material as a semiconductor film of thefirst transistor, wherein the gate wiring is formed over the film withthe insulating film interposed therebetween, wherein gate electrodes ofsaid third and fourth transistors are electrically connected to saidscanning line, wherein one of source and drain regions of said thirdtransistor is electrically connected to said signal line, wherein one ofsource and drain regions of said fourth transistor is electricallyconnected to a drain region of said first transistor, wherein one ofsource and drain regions of said second transistor is electricallyconnected to said drain region of said first transistor, wherein each ofthe first transistor, the second transistor, the third transistor andthe fourth transistor is a thin-film transistor using polysilicon, andwherein the power line is a laminate film comprising: a first layercomprising titanium, the first layer being electrically connected to asource region of said first transistor; a second layer comprisingaluminum, over the first layer; and a third layer comprising titanium,over the second layer.
 17. The semiconductor device according to claim16, wherein said third and fourth transistors are of reverse polarity tosaid second transistor.
 18. The semiconductor device according to claim16, wherein said third and fourth transistors are of the same polarity.19. The semiconductor device according to claim 16, wherein said firstand second transistors are of the same polarity.
 20. The electronicdevice comprising the semiconductor device according to claim 16,wherein said electronic device is selected from the group consisting ofa light emitting display device, a digital still camera, a lap-topcomputer, a mobile computer, a portable image reproduction apparatus, agoggle type display, a video camera, and a mobile telephone.